For best results, set SETI@home to Normal (instead of High) and everything else to Low, Medium, or High, depending on usage and behavior. Most background apps function the same on Low as on Normal (or without Peekaboo). Mozilla also works fine on Low. Almost everything else can be set to Medium. Appleworks and Finder should not be set to Low because they slow down way too much.
Why not set SETI to High? You get some of the CPU on Normal. I don't have a clear explanation, but according to SETI Logger, SETI was getting 80-83% of the CPU on High in Peekaboo, while setting it to normal gave it 91-93% of the CPU. (Same applications running, different workunits, 10 hours on each setting to filter out stray datapoints. I believe the data is good.)
Peekaboo is available via the team's Software link. It is more than worth the $20 shareware.