Actually, both DDR2 800 CL4 and 1066 CL5 can access stored information at virtually the same speed.From Here
As you will only need to run your FSB at 308Mhz to hit 4Ghz with your CPU, either set of RAM will work equally well.
Heck, you could run DDR2 667 with your intended overclock and never have to overclock the RAM.
Get which ever is cheaper or is from a better manufacturer.
Both will work equally good for you.