Don't waste your money on a new Mac if you are looking to upgrade, just get yourself a huge monitor. That's Paul Boutin's theory, and I agree. Computers now a days are pretty much powerful enough to handle whatever the regular user throws at them (I'm not talking about pro users like video editors and the like, they have known the benefits of a large monitor for a long time). You'll get the most bang for your buck with a new, crisp, big monitor.
My new 23 inch Cinema Display at work has really increased my productivity. I have a lot more room for everything I need.
Give it some thought when you're next in the market for a new computer.