I have a sapphire Radeon HD 4890. It does great for me on Starcraft 2 and Diablo 3 Beta (sorry, I don't play any of the new 3d shooters). But it lacks DirectX 11.
You can get 5770/6770's for around $100. According to:http://www.tomshardware.com/reviews/best-graphics-card-gaming-performance,3042-7.html
the cards are in the same tier, although the 4890 has slightly more raw power. But you have a GTS450, which is only one tier lower. I would definitely recommend staying with the same card or going higher as you're getting a relatively small increase in performance for going with a 4890 over the GTS450.
Probably a 6970 for ~170 is where I'd start if I were to upgrade from where you are. If you've never read the tom's hardware best graphics cards for the money, they're a great read.