I'd go with 7.1, which takes up the least RAM of any version that can run on a Q800 (this means more speed) or patch up your 7.5 to 7.5.5 (if I recall Apple has the updater on the same page you can find 7.5.3).

7.6 is a stable OS and is also a good choice, though it will use more resources. While OS 8 will run, it won't be a speed demon.