I haven't purchased a desktop machine in some time, so I can't give too much advice, but you will be happier with a non-nVidia video card. While most modern distributions have some automated install of the infamous nVidia drivers (proprietary kernel drivers, as opposed to the open X support available for pretty much any other manufacturer these days), they are still annoying, sometimes buggy, and possibly still a pain to upgrade. And proprietary.

You may also be surprised at how friendly Linux has become in recent years. Even Debian is really not that hard to install, and it's what I'd recommend, although a lot of people seem to be enjoying Ubuntu as well, which still lets you access the enormous Debian archives if you'd like. In my opinion Gentoo is all hype, but I guess I won't stop someone from trying it.