My vote goes PowerColor HD 5770 for $120, not only is it faster than the GTS 450 in 90% of games but it also requires less power.
Second would be GTS 450 if you find it for $120 or less. IMO PhysX is useless on a GTS 450 because it does not have enough juice to run both high settings + PhysX, you're better off going for pure power and high settings on the HD 5770. However, the GTS 450 has CuDA for applications and hardware accelerated DivX, Xvid and MPEG-4 which makes it worth $120 if you want these features and don't mind less performance in games. NVIDIA also has 3D which would cost you $200 for the glasses and a new 120hz 3D monitor.
In contrary the HD 5770 has Stream for videos and hardware accelerated MPEG2, H.264 and VC-1. AMD also has hardware accelerated video conversion built into the drivers and Eye of Infinity which is useless on a HD 5770 anyway like PhysX on a GTS 450. On top of all that the HD 5770 has a display-port output which is the future replacement for DVI. Edit
: Only a very small number of games support PhysX and with a GTS 450 @ 1600 x 900 you're not going to get 60 FPS max settings in these games and WoW is not one of them.