You won't see a significant difference from Q6600 to i5 750 when playing games. The design is different between the two. The i5 is a more efficient CPU and is a true quad core unlike the Core 2 Quad, which were more like Core2Duo X2.
The usage would be the same, but the time it uses should be shorter. Try running some benchmarks like Cinebench or SuperPi if you wanna see the difference.