personally i'd take a ATI 4800 series over an NVIDIA 9000 series any day of the week
but like people have said it depends on your budget
the GTX's are just too expensive for the extra performance, compared to other cards IMHO!
also i believe CUDA will work on any NVIDIA card 8 series or higher, not just the GTX cards, there have also been some nice advancements in people hacking CUDA to work on ATI cards, so don't get a GTX just for the added benefits of CUDA or PhysX
J.R.