I am looking at a graphics upgrade from my Gigabyte 3850 256MB. For a similar price (I think the Nvidia is slightly more expensive) I can get a HD4850 512MB made by XFX with the non-reference fansink or a Nvidia GTS250 512MB.
I have been looking at reviews that seem to show the ATI being sometimes better and the Nvidia being sometimes better, however the reviews I have found thus far are all pitting a 512MB 4850 vs a 1GB GTS250 and I can't afford the 1GB.
If they are around the same performance or if it is a question of preference I think I will get the 4850 - better brand, longer warranty and I was unhappy with the last two Nvidias I have owned. I have laptop that is affected by a round of shoddy Nvidia laptop GPUs and want to avoid them if possible. That said if the Nvidia is a lot better and will continue to be better as more games come out I will give them another go!