What is the monitor resolution? The 465 tends to trail the HD5850 at higher resolutions with them being fairly equal at medium/lower settings.
Me, I'd go for the 5850 (and have done) because of its quieter operation and lower temperatures, but ATI are not getting it all their own way in that respect:http://www.techpowerup.com/reviews/MSI/GTX_465_Twin_Frozr_II/
But then again, hate that type of cooler, but you might like it.
If PhysX/Cuda is importaint the anwser is obvious, though