make sure the BIOS is giving the ram proper voltage, which appears to be 2.1v from a quick google.
4, 2 gig sticks, right? But this is vista 32 you say? 32 will not support more than 4 and you should not be seeing more than 3 with your set up. What gives?
Sometime putting 4 sticks in a mobo entails a few non-standard tweaks - such as relaxing timings, reducing ram bus speed, and sometimes a slight voltage bump.
Try 2. 1 v first, then if that does not fix it look into bus speed or timing adjustments.