What kind of power supply do you have in the PC?
edit: Actually, it'd also be very handy to know the CPU, motherboard, RAM, possibly OS, and definitely monitor (more to the point, the resolution(s) you'd most likely be wanting to run things at). Oh, and any other games / graphical apps you might be thinking about running.
edit2: Just in case it's troublesome to answer the above, or you simply don't want to for whatever reason, I'd recommend the GTS450 - like say,
this one (under $100,
after mail-in rebate, mind you). That's an excellent GPU for the money atm, provided your PC (particularly the PSU) can handle it, and can otherwise match it, or close enough. But yeah, you can't do better at that price point, that I've seen.