The GeForce FX 5200 is a very, very bad videocard. That's why Thief 3 runs like crap.
For under $150, the best deals are an NVIDIA GeForce FX 5700 Ultra 128 MB and an ATI Radeon 9600 XT 128 MB. Other choices would be any NVIDIA GeForce 4 TI series. You can find TI4200's for under $100 now, and NewEgg has refurbs for about $70.
If you want to spend $170 you can find a huge increase in power with the NVIDIA GeForce FX 5900. If you want to spend right at $200 you'll have an even bigger increase in power with an ATI Radeon 9800 Pro.
EDIT: I know you say you want a videocard that will last a while, but that won't happen unless you spend at least $170. $150 will get a damn good videocard, but it won't be top of the line, and in a year or so you'll have to upgrade again.
-Lasereth