Turn off all Extensions you don't need, turn down the Disk Cache. That's about it.

If you're on a machine with a PMMU, turn on Virtual Memory. If you are on one of the first-generation Power Macs (the only PPCs that can run 7.1,) definitely turn on Virtual Memory. Regardless of how slow the hard drive, PPCs like Virtual Memory to be on, unless you have an insane amount of RAM. (32 MB for 7.1, 64 MB for 7.5-7.6, 128 for 8-8.6, 256 for 9.)