1) performance increase will depend on what resolution you're gaming at, what games you're playing, and how high you OC it.
2) quiet/noise doesn't affect overclocking ability
3) various programs such as NTune/NVTweak, coolbits, ATI Tool (yes, ATI with your Nvidia)
The difficulty is that overclocking video cards.......it generally depends on the model. The 7600GT can be overclocked respectably without any special volt mods, but here's the catch: push it too hard and you can burn it out. Unlike a CPU which can easily shut down, you might not know there's a problem until it's too late. OC'ing a graphics card is easy, but knowing if it's safe is very difficult. Not worth the risk in my opinion.
Up to you, but there's some info for you to chew on.