I did the exact same thing a couple of months ago. I started a thread on which I got some excellent advice on which distro to go for, so I advise you read that - http://blenderartists.org/forum/showthread.php?t=156463
I use 32 bit Ubuntu for daily stuff, or 64 bit if I’m doing something that geninely requires the extra RAM (32 bit will only recognise up to 3Gb). 64 has a few issues with that make it a little annoying for daily use, but it is great for blender.
Ubuntu is a great distro though, highly recommended.
Hardware wise I agree with Nero about nVidia, historically ATI has had poor support for linux that I’ve been burned by in the past. That said RAM and processor power will give you the greatest bang for the buck with blender, so go for a mid-range graphics card (I paid under £100 for mine and it’s doing an excellent job), 4GB RAM (or 8 if you really want), and then the best processor you can afford - that will make the biggest difference, and should certainly be the most expensive component you buy.
P.S. If you’re lucky Michael W will weigh in on this. He knows his stuff.