Get the Radeon 9600 se. Between nVidia and ATI's latest cards, ATI's are significantly faster and cost the same price. nVidia's drivers are easier to install (it's just one file no matter what card you've got), but that's about it.
You won't be disappointed with either company's products, but facts are facts (I read PC Gamer's hardware reviews religiously), and ATI's current generation of cards are faster than nVidia's. Go buy the Raden 9600se if you've still got time to.