well at the end of this year I'm going to go Quad again. But I want a chip that can hit 4GHz on air and maby a little more. My question to you is. Should I buy the E8400 or the E8500? I have the E8400 in one machine but it is CO stepping and I want a EO stepping so how do I tell if it is EO and not CO? I would love the E8600 but for the price I don't think it's worth it.
If your goal is 4GHz it may not matter E8400 vs. E8500, but I would start with the E8500 just for the slightly higher multiplier. That way it's a little bit easier on the ram and the mobo. Consider that the E8500 with the mobo set to 400MHz is running at 3.8GHz, and this is right at the rated speed for 800MHz memory.
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.
i got my e8400 up to 4.140 on air (the temps hti 70 under load which is a bit hot, but fine by me). As has been said it depends on the chip but chances are you will be able to get both the e8400 and the e8500 above 4GHz on air