Ah, got it....
My system can run 1066Mhz, 1333Mhz, or 1600Mhz (Intel i7-950). I've run all three speeds, very little if any difference....(& yeah, 1600Mhz is touchy so I always run 1333Mhz....)
BUT, I've run AMD Phenom II X2 550's vs AMD Athon II X4 630 no contest, the four (4) cores really make a Noticeable difference.... (and Phenoms are 15--20% faster than Athon's...)
4GB vs 8Gb is supposed to be a bit faster too.... (haven't run this myself I run 6Gb & there isn't supposed to be much of an advantage to goto 12Gb, so, I haven't...)