RE: .."difference in performance between the 2.8, 3 and 3.2 machines"
Generally, the difference is slight when it comes to processor speed. The higher ghz machines sell better later on.
Get the minimum when it comes to harddrives and ram and go with aftermarket, non Apple (never apple products anyway, they just charge a premium) and put in fast drives 7200+ and grab your Ram from Crucial (where Apple gets it) or other reputable dealer. I can't really comment on graphics cards other than you get what you pay for. Do a little research for your specific needs. Most cards and talk of them is geared toward gaming rather than professional use so you'll have to weed through.