I've owned an nVidia FX 5200 until about two months ago (yeah, go ahead and laugh, it's your right), until I purhcased an ATi x1600 Pro 512MB from Newegg. I haven't had one single problem with ATi's drivers on XP. It works wonderfully with my dual-monitor setup, and the graphics in Halo, Battlefield, and Unreal are just...unreal! (Although I do think I am going to see another jump in performance after I upgrade to an AMD processor and a 450W PSU...250W probably isn't helping.)
You won't lose either way, as I see it. I'm not really a fanboy of any company, I just use the best I can for the best value I can get it at.