I've seen used 750 Tis go for less than 100E on eBay, but not often. 100E is certainly well more than enough for a good 560 Ti
(by that I mean 900MHz core minimum) - heck, I bought one recently for only 45 UKP, which is about 55E. 100E is also enough
for a GTX 580 1.5GB (faster than all those options), but other posters are right, without knowing the PSU one cannot confidently
recommend an older upper-end card like a 470 or 580. This is why, despite the higher cost, a 750 Ti is a safer choice as it would
not require a PSU change.
However, we're overlooking the platform too much. With a card as fast as a 750 Ti, the E5700 could easily be a bottleneck in
many cases, though of course later on the original poster could replace the base platform & carry over the 750 Ti & observe
a good speedup from a better CPU, etc. Even a well-chosen used-parts purchase would achieve this, eg. i5 2500K, Z68 board, etc.
Ian.