Depending on how many plug-ins and audio tracks your gonna record, you wil either need a mac tower (G5 or mac pro) or you will be perfectly fine with a second hand G5 imac - which i use (but isnt second hand) iv put as many plug-ins and synths into logic as i humanly can to test it, and i managed a shit load of stuff, remember - just cos its out and stupidly fast it is - doesnt mean you have to have it! G4's are still powerful machines, the 1.25 ones anyway, my mate uses one and he has no problems with audio and mid work.

If i had to choose though, and i had the money - id definatly get one of the new mac pro's.