Unless you are running things that require more than 2gb, I would go with the faster
RAM. Here is the logic....
1066 > 800: Small difference in
FSB may not make a noticeable difference. 266mhz is not small. As a matter of fact that is over a 25% increase in the speed of the
FSB.
BIOS: Depending upon the BIOS of the motherboard you may not be able to have the
RAM run at one speed and the
CPU and rest of the system at another. Therefore using 800mhz
RAM may limit the speed your
CPU can run at. Remember that your
CPU speed is (
FSB/4) * Multiplier. So with a multiplier of say, 10, you are talking about a
CPU difference of 2ghz ((800/4 * 10) = 200 * 10 = 2ghz). With a 1066 bus you are looking at 2.66ghz ((1066/4)*10=266*10=2.66ghz).
Corasir > GSkill: THis one is my opinion, but I am sure many will agree. If you were talking OCZ, Mushkin, Geil, or Crucial the difference might be arguable.
finally, if the 4gb is 4x1gb then it will probably cause issues with the DDR part of it. It is a known fact that motherboards that support DDR run better with only 2 sticks, especially if you plan on overclocking.
There is My Two Cents, take it for what it's worth.