I have used Nvidias Geforce cards all the way back since the original Geforce 256, as it was called. I have never had any problems with them, drivers always a pleasure to install with the Detonator packs ( Now called Nforce, or something ) and never reflected that ATI might be better.
Now, with the purchase of my Dell computer, I have a ATI Radeon 9800 Pro and I found out it is just as userfriendly as the Geforce.. So I could swing either way if I was to get a new card.. :-)
Another thing to consider is that the high-end Geforce cards usually have such large fans that they occupy a PCI slot aswell, only to allow the heatsink/fan to fit, whereas the ATI don´t need as much cooling ( I figure )...
