If you have an installer for 7.6.1 (it's not a free download from Apple) and a machine capable of running it, 7.5.x isn't even in the running. It's a buggy, crashy mess. 7.6.1 is far superior in just about every way.

8.1 is also nice, but maybe a little top-heavy on an 040. Give it plenty of RAM if you do use it.