Either Nvidia or ATi should be choice in brand. You need to do some research into how much video power and how much money you are looking to spend.
Top of line Nvidia is the 6800 Ultra around $500, same for the ATi X800 pro platinum. Lower versions of both brands are available at many varied prices, from $50 on up. Of course the more you pay, the more powerful the card, at least most of the time.