Should help to confirm the decision:
http://www23.tomshardware.com/cpu.ht...=433&chart=174
That's at stock (the E6400 and E4400 are very nearly the same speed).
When overclocked you're looking at something like:
http://www23.tomshardware.com/cpu.ht...=430&chart=174
And that's if you can get the 4200+ to 2.6GHz.