Every processor will vary, but it seems that 4.2 or 4.3 on air cooling is pretty much the limit that you could expect. I bought the E8500 for my office and didn't want to spend a lot of time tweaking/stressing/testing it so I just set it at 3.8 right off the bat and it runs fine. Not many people will tell the difference between 3.8 and 4.2 anyway, although we certainly can with our CAD apps. If you want to go to those speeds you might even consider water cooling.
I know very little about overclocking RAM but you might be able to get to 4 or beyond with your 800mhz ram.
My machine at 3.8GHz ran SuperPi1.5 1M in