I can't tell you how much is the max, because I, with a G4 800MHz, never reached the max. I can only tell you about my set-up and experiences so far. I have a RME Hammerfall Multiface, a TC-Works Powercore-card and Logic Platinum 5.5. It really works like a dream. My advise is to go for the fastest system you can get. Just ask for info about using duals with audio. I've heard there can be some problems. So, maybe a single G4 is better, but I'm not sure. I would buy the maximum RAM you can get. I would choose Logic, because it's always been ahead of Cubase and now especially, because it's an Apple company, so integration with MacOS X is perfect and will be even better in the future. If you want low latency, go for Mac, not PC. MacOS X is THE music-OS. There's nothing that can compete.
Good reverbs have always been too heavy for Mac's/PC's, so that's why I bought the great Powercore-card, that has wonderful reverbs, compressors, EQ, etc., with no burden on your CPU. Also look at the wonderful UAD-1. Which is very good as a compressor, but has more wonderful things. I mainly use the Emagic EXS24 Mk2 sampler as my sound source (no hardware) and some soft-synths.
Logic 6 will be out this month, which is wonderful. Powercore MacOS X drivers will be released any day now (maybe they're already there). As a sound card the RME Hammerfall Multiface is great. Check their site. Great sound and latency. Also take a look at MOTU's Firewire sound-cards. There's more. They're OS X-ready.