Nvidia linux drivers are generally considered better than ATI video drivers for linux. Nvidia fairly regularly updates their proprietary (binary only, no source code) video card drivers so you can actually get a reasonable amount of control over your vid card running linux if you use those drivers.
A linux "purist" still hates both ATI and Nvidia for not releasing full source code to all their drivers, but people who just want to *use* linux and not make love to it don't seem to care much

That said, if you're not playing games or using one of the spiffy new 3D enhanced desktops on your linux box then it doesn't matter too much what vid card you're using. But if you want the latest in eye candy or want to play games on the linux box, you're much better off with an nvidia video card. Even the integrated nvidia cards such as the 6150 based chipsets are fairly well supported by the nvidia drivers.
I've been using ubuntu recently and it's pretty decent. Just remember like almost any OS, you'll probably want to update the vid drivers after installation because it doesn't ship with the latest nvidia drivers. They're listed in the software download/update tool that comes with ubuntu, so it's really easy to do.