Overclocking memory on GIGABYTE GA-G31M-ES2L

What CPU are you planning on OC'ing?

e5300 2.6ghz

Well DDR2-1066 is just overclocked DDR2-800 really, but I usually pay the premium for the 1066 RAM just so I get the guarantee/warranty it'll definately work at that speed. Up to you  

Edit: nevermind, just get what RAM you want, your stock FSB is only 200Mhz, so to get to 4Ghz you only need 307Mhz FSB (which you can do with DDR2-667). To make use of a speed higher than 667 you'll need to use a memory multi (being a gigabyte it probably has them), so it'd be up to you. 307 * 2.6 = 800, so DDR2-800 would be good if you've got a 2.6 multi.

The E5200's seem to be better overclockers.

You did not mention the cpu that you are going to use.
I am using E5200 and have pushed my ddr2 533 MHZ to 810 Mhz........lol with 2.6 multi.
at 3.8 GHZ i see that Cpu setting is that i need to tweak becasue ram seems to be fine ( a gut felling and memtest gone good )
Not able to boot windos at 4mhz but that seems more like a cpu voltge issue.
Damn don even have a new heat sink and i need it badly...but seriously i can see the performance gain very well.
Thank You.