I have a Core 2 E8400 clocked at 3.6GHz, with 2 gigs of Kingston HyperX 1066 Ram. That puts my fsb at 1600 (400 x 4), so my ram to fsb ratio is 3:2 (533:400). Should i leave it there, or set it to synced and bump my ram down to 800, which would run it 1:1. (Btw when i run it at 1066, i set timings to 5-5-5-15, and 4-4-4-12 in 800) Which would be faster / have better performance results? I run an XP / Vista mix.

It will be negligible difference either way. If you want a simple test, choose a game and do a benchmark with the ram at its best possible attainable timings at 1066MHz and again and 800MHz, and see which performs better, and then use that as your setting. I'm guessing you could get below 5-5-5-15 @ 1066 and 4-4-4-12 @ 800 if you played around with the ram timings a bit.