If you're not sure how powerful your PSU is, then i'd recommend something new and low-end, it's always safer. As new lower-ends perform as well as older mid-to high-end cards used to.
For exapmle GT640(better variant, not the 144CUDA core one) has the same amount of CUDA cores as 560Ti, only with weaker memory bus, making the performance a bit worse. But it still should do as good as 9800gtx+... Okay, it's not as good as gtx9800, but still... its good enough to play games http://www.hwcompare.com/12940/geforce-9800-gtx-vs-geforce-gt-640-ddr3/