I finally got around to testing my piano overload song on my other Mac, a dual 1GHz G4 tower with 768MB of RAM, booted off of a 4200 RPM external FireWire drive. The second CPU more than made up for the slower hard drive and lower memory, allowing me to add four more software pianos than I could use on the PowerBook.
Final total: nine software pianos, five software percussion instruments, a software acoustic bass, and seven sampled loops. I’ll try again when I finish rebuilding the fast internal hard drive, to see if that will let me squeeze in another piano.
Update: Nope, the faster hard drive didn’t make a difference. It looks like CPU and RAM dominate, and dual CPUs make a big difference (as they should).
Update: Past a certain point, more RAM doesn’t make a big difference, either (says the guy who just bumped his PowerBook to 2GB). It looks like 512MB is a good working minimum, and 768MB or so will handle just about anything a G4 or two can keep up with. Above that, you’re basically adding buffer cache to compensate for the speed of your hard drive.