I've always used Nvidia, but I have read good things about ATI and think either would be a good choice as long as it has the capabilities your game requires.
yeah I have always been a Nvidia user, but my friend has always used ATi. We game togethre alot and I have never heard him complain about his card. So either seem to be pretty good
I've used both kind of cards. As a Linux user I much prefer Nvidia over ATI, mainly for the driver support than anything else, although I did use ATI cards for a long, long time and only recently swapped over to NVidia based cards.
The current card I'm using is a EVGA 260 GTX 896MB Core 216 Superclocked, and it runs like a charm. It's a beast in size though, big and chunky and takes up lots of space inside most cases.
I've had so many problems (overheats, incompatibilities etc) with ATI cards in the past both on Macs and PCs, both desktops and portables so Nvidia is my choice for the near future.