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.