I would stick with Nvida all the way .. Go for the radeon but only if you plan on getting the 9800 pro. If you plan on staying midd range get a ti4600 or something check http://www.GoogleGear.com great prices even better service and 2 DAY shipping for only 50 cents.. Ordered from there MANY MANY times no problems at all
Oh, and I wouldn't bother with getting a 256MB card. It's just a waste of money. No current games or near future games will use 256MB of graphic memory. Even Doom 3 will only use 90MB. You may get a few extra FPS but it's not worth the $100-200 extra.