Well, I've found the e6550's are pretty hardy little things, easily overclockable to 3.2Ghz
The thing that's bottlenecking your system atm, and it will be even more noticable if you got a q6600, is your 8600GT.
Myself, I'd upgrade to the 8800GT instead of upgrading the cpu. Though Vernong made a good point about your psu.
edit: I did a psu calculation, and your psu should be fine. (depending on how much amperage it can supply)