I prefer ATI to be honest.
I have a 8800GTS 640MB in my computer at the minute, but I really miss my Radeon 9700PRO, which was one of my first major graphics cards I bought, other than the Nvidia Geforce MX400 I had (lasted ages)
I'm wanting to get a 4870 1GB though at some point, but by the time I actually come to get one, another will probably be out that'll be a better mid range card.
I guess its a bit pointless though coming from me, because I find ATI has better support for 3D packages, where more than one 3D display has to display at once. I get flickering in menu's (menu's become invisible randomly) quite a lot.
Its not the drivers either, as I have updated in the past to no avail.