The key is what will you be doing with this computer? If you use apps that utilize the third core than go for it and get yourself the x3. If your just gaming get a different core 2 (ive got an e7400) with a higher mulitplier for overclocking. If you dont plan on overclocking alot then get yourself an OEM 9950 which will easily go to 2.8-2.9GHz for about the same price and has the addition of a fourth core and isnt built from defective quads. see: x3 series.
I REALLY hope you're not telling him to get a first-gen Phenom. They're terrible compared to Phenom IIs. I believe I saw a benchmark somewhere where the 720 beat all of the first-gen quad cores, despite having 1 less core.
@OP, I'd take the 720. While the E8400 has higher clocks, the 720 has an extra core to run all of the background programs, leaving 2 for nothing but gaming. It also outperforms the E8400 in programs that take advantage of more than two cores.