i run renoise on my g4 powerbook (886mhz) and the number of effects/vst’s i can use is somewhat limited. i can certainly be productive though, it just takes a little brainwork to see how many effects i can fit in before things get overloaded.
the mac os version is supposedly not very optimized yet, so performance should improve in the future.
definitely get a ram upgrade to 512M and consider the 1.42 ghz model if you like to work with lots of effects.