|This is where it get's confusing. Intel "quad-pumps" the FSB. So 800MHz FSB actually runs at 200MHz. To be in sync at 1:1, you need to run the RAM at 200MHz (DDR400).|
To make things even more confusing, the P4 & it's netburst architecture is very inefficient, so to get the best performance out of a P4 system, the RAM freq should be faster than the CPU freq. Either 3:4 or 4:5 depending on the CPU.
For a Core 2 system, testing has shown that running 1:2 ratio performs equally as well as 1:1 IF you can pull it off. So an 800MHz FSB Pentium dual core (Core 2 based) with DDR2-800 RAM would give you the 1:2 ratio. A 1066MHz FSB Core 2 Duo with DDR2-1066 would also give you a 1:2 ratio. But since many Core 2's will overclock to 1800-2000MHz FSB, you would most likely have to drop back to 1:1 again.
The original Pentium, P2's & P3's should always be run at 1:1...ditto for the AMD K6, K6-2, Athlon & Athlon XP.