ram issue

windows xp i have
1 512, 1 512, 1 256. should equal 1280, windows says 768,
mother motherboard, cpu-z, pc pitstop all see all 1280, but all say windows is only using 768. i tried bios flash, reset cmos, reinstall xp drivers and all, nothing helped. I know all 3 ram are working and are the same type. my mother board is a a7v8x-x and says up too 3 gigs, ddr 400, 333, also says ddr400 max. 2 banks, ddr333 max 4 banks up to 3 gigs on either.
i took my pc to 2 stores they dont have a clue. anyone have any ideas?

I know

Your DIMM 0 slot has the 256 in it, your DIMM 1 slot has a 512 in it, and your DIMM 2 slot has a 512 in it. What's happening is that the first and second sticks (your 256 and your 512) are both double-sided which effectively "take up" four slots.

Want an example? Download Sandra, run the memory benchmark, and then scroll down to see how Sandra reports the slots as being filled. I bet you see this:

Slot 1: 128mb
Slot 2: 128mb
Slot 3: 256mb
Slot 4: 256mb

Double sided DIMM's are a bit misleading because of this oddity. But I'd bet this is exactly what you're up against...