Never buy a processor based on what it *might* do if you *manage* to overclock it. There's plenty of people who've got the X3 720 to 3.6 - 3.8 GHz on good air cooling at which it undoubtedly outperforms a stock-clocked E8400!
Compare the two processors at stock - using
AnandTech's beta CPU benchmarker - X3 720 (blue) vs E8400 (orange). Whether a higher or lower score is better is test-dependant - look through the tests carefully and you'll see a lot more wins for the X3 720 than for the E8400.
Add into that the good overclocking potential of the X3 720, the fact that it's £20 cheaper, and the potential to unlock a 4th core on some motherboards, and it becomes a bit of a no brainer, IMNSHO!
Also, don't forget that as multi-core processors start to inflitrate the mainstream (if / when AMD release Phenom II derived processors with no L3 cache this will happen quite quickly!), a lot more progams will be designed to use those extra cores - and the X3 720 will be waiting there to lap them up!