hmm... using Rambus... well, thats points to alot of the possible problem right there. Rambus has many limits to it. It must be in matched pairs, also make sure if there are anymore RD slots, you have it properly terminated. Also, at one point in time MS had an issue where Rambus would only be recognized upto 512 meg of memory, you may want to see about windows updates as well as bios updates. It sounds though like you have a problem with matched pairs. You, I'm guessing, have 2x256 and are adding 1x128, well you need another 1x128 or (not sure if its available this small for Rambus, I don't think it is) got a 2x64. Syntax, think you need to work on the social skills a bit.
EDIT: Sorry DJChris, just now realized you pretty much posted the same thing.
If you argue with an idiot he will drag you down to his level and beat you with experience.
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