2Gb works 4Gb doesnt and 6Gb only registers 5Gb..

[eternal kaos]

A bit wierd but my computer is running with 6gb of RAM in and only registering 5gb of it on start up and registers 6gb on CPUz. On my sidebar (vista ultimate) it says 5gb as well. Using 2gb of crucial ballistix and 4gb of corsair xms both 6400 (yeah i know dont mix ram) but seems to work. When I drop it to just the 4gb of corsiair my computer wont boot into windows then after a restart wont post.
It runs fine with just the 2gb crucial in and again runs fine with each individual stick of corsair in but when you start mixing and matching it messes up... unless you put it all in. Any suggestions? apart from build a new system...

This is using:
MSI K9A platinum
AMD 6400+ black ed
Gigabyte 3870
and the ram...
also vista ultimate ed

 

[daship]

Get matched ram.