Gah! If only you asked a week earlier! I would've pointed you at an EVGA GTX 460 that was only going for $129.99 on NewEgg! Anyways, according to TomsHardware's official Graphics Card Hierarchy Chart, then the HD 5770 has one rank above the GTS 450 and if they are going for same price, then you're probably better off picking up the HD 5770.
this is a close one but for FYI, the GTX 460 768mb version does beta the GTS 450..
GTS450 and HD5770 are close in competition with the HD5770 by a SLIM nose.
the GTS450 does beat the HD5750.
with that being said me personally will take the GTS450 over the HD5770 I favor nVidia and CUDA/physX.
and in SLI the GTS450 actually matches the HD5770 in it's X-Fire mode closer than single cards.!
and I'm not so fond of PowerColor..
I run a Phenom II X4 940 with a MSI GTS450 Cyclone..
Of course the GTX 460 768MB is a lot faster than the GTS450 or the HD 5770. But id don't know if the OP wants to spend another 50$ or not.
the 5770 is faster than th gts 450, particularly in higher resolutions. If you plan to add a second card to sli/crossfire in the future, i would suggest the gts450 though as it has better multi card scaling.