Better lens and post capture processing
I don't know about the Rebel XS. I don't even know about Macs! I'm only going to get the ball rolling with some general points. You're going to get a big imac, but write about cost-effectiveness. If you were really worried about that, you'd get a PC. People will find it difficult to advise you unless you make your photographic interests more specific so they can 'solve' your problem.