E8400 is a far better investment than the Q6600 for most users but it all depends on what you use your system for, how much improvement you see. I would buy the E8400 over the Q6600 for most users out there within a heartbeat! 4 GHz E8400 is monster performance we wouldn't have dreamed of possible this easily a year back
3.8 to 4GHz is what is usually possible for regular use, yes.