If you don't want to spend much, pick up this $82 GTX 460 768MB: http://slickdeals.net/forums/showthread.php?t=3583262
If you do want to spend a bunch, get a GTX 560Ti for $200. It's your call. But the GTS 450's pretty much a sidegrade, not an upgrade.
If you have $200, you'll end up happiest spending $120 on a 22"/21.5" monitor and $80 on a GTX 460 768MB.
Then overclock that graphics card to really get your money's worth. That'll be quick and easy with MSI Afterburner/Kombustor. PM me if you want to start a thread on graphics overclocking and I'll walk you through it. It'll only take 30 minutes if you know what you're doing for a 20% performance bump--and 2 hours if you don't know what you're doing.