Yes, it can be done, but it will take a little work! If running win98 or later, you can use multiple video cards.
I have run a 98 machine this way for some time right after 98 came out, just to see if it would work well with games. It worked okay for me then (so I would imagine that the games would work better now, but who knows?).
First, you have to have video cards that will
A) work with windows properly to do the dual card
B) work with each other
Next, you have to have at least one that will give you good performance on your games (obviously

). Finally,
most games will default to your primary video card in the system, and they will not let you change it--so your primary card has to be your gaming card.
Assuming that you meet that criteria, and you get it to work, you might notice a performance hit. Some people report big performance hits, some don't notice at all.
That being said, some games are actually able to utilize multiple video cards/monitors to display more screen area, game stats, and other options. So some of your games will probably not care about another video card in the system. However, I would not be surprised if some of your games just threw a fit!
This is just my take on it!! If anybody else has thoughts on this one, feel free to jump on in here! Mudskipper
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Groucho said it best- "A four year-old child could understand this! Quick! Run out and find me a four year-old child: I can't make heads nor tails out of this!"