How to buy a graphics card, short version.
Neither brand is better. They both make good cards at various price ranges so depending on how much you can spend and what sales there are it could easily be either brand that has the most powerful card you can afford.
Check your motherboard, if it has an ATI chipset lean slightly towards ATI cards and if it has an Nvidia chipset lean slightly towards Nvidia cards. Note though you will want to uninstall and disable the drivers for the integrated video card if it's a different maker than your dedicated video card although you could potentially get both working it's not worth whatever problems might come up.
More importantly though, if the motherboard supports crossfire but not SLI, lean heavily towards ATI cards, and if it supports SLI but not crossfire, lean heavily towards Nvidia cards.
Other than that just browse newegg and whatever other sources you plan to buy from for video cards within your price range, pick out two or three that look like the best, and google up some performance comparisons and reviews to find out which is the most powerful.
Generally it's much more economical to buy a single card than two cards unless you have a really high budget, because you can save a lot of money when you make your next upgrade by making that upgrade a second copy of your single card to use SLI or crossfire, and not have to get rid of the single card. Also note that having more than 2 cards in a crossfire/SLI setup is not really a good idea yet because many current games and applications will not be able to take advantage of the setup properly.
Also check the power consumption of your card, you may need to upgrade your power supply too to support the more powerful ones, especially if you want to do an SLI or Crossfire setup.
If your price range is around $200-$230, a GTX 460 will be your best bet.
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