I dont know if this still applies with the i7s but if I buy 2 more sticks (total to 8gb, 4 sticks) would it kill my overclocking potential?
In your case, I'd just go for 2x4GB, because DDR3 is so incredibly cheap. ZeroCool was in a different situation because he was buying DDR2, which is twice as expensive.
I had the same concern when I went to 8GB from 4GB, and for that very reason, I bought 8GB, knowing that in the worst case I'd have 8, in the best case I'd have 12.
I found two things:
(1) Going with all four slots populated required a bump in IMC voltage
(2) Either due to all four slots being populated or due to the mixed sizes, the 12GB setup, while stable originally, flat out did not work when I went crossfire - I basically couldn't even get into Windows. Maybe it was because voltages were thrown off, maybe it had something to do with shared memory allocation, but I wasn't going to find out. I pulled the extra sticks.
So long story short, while I haven't tried to hit 4GHz on my i7-860, I wouldn't count on all your settings working as they do right now at that speed. Just buy 8GB...