Video Card Problems/Dual Graphics Cards?
QUESTION: Is it possible for a motherboard to support two graphics cards at the same time. I own a Gateway DX PC which cam installed with an AMD Radeon graphics card. Is it possible to install an Nvidia graphics card on the motherboard's spare PCI port and switch back and forth between them as needed as there are multiple games that are optimized exclusively for one brand over the other?
ANSWER: If the motherboard and power supply can handle the expansion cards you'd like to install, yes you can install whatever expansion cards you'd like. However physical/electrical compatibility isn't the end of the story. The other side of the coin is software - you will need drivers for the cards you're installing, and depending on what operating system you use, this may or may not be a possible configuration (Windows Vista, for example, will not allow this to happen, while Windows XP or Ubuntu Linux will have no problems).
It should also be noted that PCI is not a good choice for a graphics controller - it lacks the bandwidth to properly serve a modern GPU, especially for 3D rendering scenarios (e.g. gaming). Cards that exist for the PCI interface are generally very dated (most do not support anything beyond DirectX 8), and generally serve a niche market for machines that lack AGP or PCI Express and need to support more than one monitor.
Regarding the second part of your question - this is not feasible. While some games will allow you to select a rendering device, each card would need its own associated display, and there is no performance benefit to doing what you've proposed (no games are "exclusively optimized" for nVidia or AMD graphics - what you may be noticing is that some games licence or otherwise share technology with one of those companies (both nVidia and AMD actively develop 3D technologies and features beyond physical GPUs), and in some cases there may be higher-level features that are exclusive to a given manufacturer (such as nVidia PhysX, however one should remember that the supported software list for hardware-enabled PhysX is extremely short), and these features do not detract from core gameplay (they're often just "extras")). If you're experiencing poor performance in a game, it would likely be better to upgrade to a more powerful graphics adapter overall - in order to better comment on this I would need to know much more about your computer and its configuration, and what titles you're meaning to play.
One historic exception to the above is 3dfx GLiDE, which is a separate API (similar to OpenGL) developed by 3dfx in the 1990s to improve performance in certain 3D games. Only 3dfx hardware is capable of running GLiDE, however this all predates the popularity of DirectX, and the majority of GLiDE games can be run in OpenGL as well (and given the massive performance improvements that have occurred since the 1990s, running such a game in OpenGL on modern hardware will result in better performance than a contemporaneous 3dfx card with GLiDE). In the case of a few specific games that require GLiDE, the Voodoo 2 is a popular add-in (and will work exactly as you're describing, without the need for a secondary monitor), but this is a very specialized configuration (in other words, if this is all Greek to you, you probably do not need this kind of configuration).
---------- FOLLOW-UP ----------
QUESTION: You said dual graphics cards won't work on windows vista but will on windows xp or ubuntu what about windows 7 ?
ANSWER: Windows 7 will generally support such a configuration, as long as you aren't trying to use different versions of the same driver (for example many older AMD cards use a "legacy driver" which will not concurrently install with a "modern" driver; or trying to install a GeForce and Quadro (or Radeon and FirePro) in the same system). It's somewhat situational. In general I do not recommend such "mixed" configurations due to the various driver oddities that may be encountered (it gets particularly hairy when you get into GPU-assisted video decoding). Also keep in mind that various vendor-specific features will disable themselves if they detect a "competitor" product; for example PhysX is unavailable on any system with AMD Radeon graphics drivers installed, regardless of hardware support by a GeForce or Quadro part.
Again, I would not suggest buying a PCI graphics adapter and hoping to use it for gaming - it will offer no advantages (regardless of what chipset it has), and will also probably force Windows 7 into "Basic" display mode (it will probably not support Aero, or will not support Aero well, and Windows 7 will respond to that). If you can provide more information about your computer and specific needs, I can provide a more directed answer to address your concerns.
---------- FOLLOW-UP ----------
QUESTION: If dual video cards is not a good idea what type of graphics card would you recommend for gaming and movie watching AMD or Nvidia?
Both nVidia and AMD make very competent graphics adapters, and either can be a good choice. It isn't really a "cut and dry" or "black and white" kind of question - they both offer fairly diverse product ranges, and have many competitive products. For example nVidia's GeForce GTX 680 is a very powerful graphics card, and handles most games quite excellently, but so does AMD's Radeon HD 7970. They're competitors; one is not head and shoulders supreme by virtue of being nVidia or AMD based. Choosing between the two makers is often a matter of additional features (for example, if you want Eyefinity, you must go with AMD, if you want PhysX or 3D Vision Surround you must go with nVidia), or pricing (one may edge the other on pricing at any given period). If additional features and budget are not a concern, personal preference can be a valid factor, but ultimately it could come down to a coin flip as well.
Movie watching is a bit more contentious - if we're talking about DVDs or online streaming services like Hulu, it really doesn't matter, because the graphics adapter has nothing to do with that. If we're talking about HD-DVD, Blu-ray, or other h264 video that can be decoded by a GPU, it only matters inasmuch as you select a controller that supports this feature (most nVidia cards from GeForce 8 forwards and most AMD cards from Radeon HD 2000 forwards will support this functionality, and there is no performance/quality benefit to using a "high performance" model for video playback; the extra performance benefits in other venues (like gaming or GPGPU tasks)).
As far as what you should specifically buy, it depends on what games you'd like to play (different games have different system requirements - look at the most demanding game you play as your "baseline"), and what your system can support (what kinds of expansion slots, cooling, and power capabilities does it have?). From there you can determine what you "need" relative to what you can "support" and develop an idea of what kinds of products will be most appropriate for you. If your machine is very old, or has a low capacity power supply or inadequate cooling, it may require an upgrade to a new machine to support a more robust graphics card. Depending on your budget and the software you'd like to play, this may or may not be worthwhile. Another consideration in this event would be a gaming console, as they generally offer a much better performance-to-value proposition, and can play a wide range of modern games.