Best bang for your buck video card: ATI or nVidia?

I'm looking to upgrade my aging 7800GT to something a little more updated, specifically something I can play L4D2 and source based games on high with at least ~60fps. My friend recommended the 9800GTX+, but I've gone with nVidia for a while and was wondering whether ATI might be worth looking into.

I looked through the Passmark scores and the Diablotek Radeon 4830 looks even better than the 9800GTX+ and much cheaper. Any thoughts?