I was running two 2x512 A-Data Vitesta at 800 at 5-5-5-18 (this is on the
AUTO setting in BIOS - there are no Manual config changes) on a dual boot XP
and Vista Ultimate machine and NEVER EVER had an issue with this config.

I just added two 2x1024 A-Data Vitesta at 800 so all banks were full. The
first and third (yellow) banks are 512 and the second and fourth (black)
banks are the 1024 and the timing stayed the same 5-5-5-18.

I was running XP without issue until I started playing a game
(Counterstrike) and the game would freeze briefly and then it would come out
of the freeze but I never have a problem with the XP O/S and never have had a
problem with other games (WoW and DDO:Stormreach) work fine. I tried Vista
Ultimate and within about 120 seconds the entire OS would lock up. I took
out all but one stick (I thought the 1024's were bad) and tested it. NO
ISSUE. I ran a Vista's Memtest and the 1024 stick was good, so I did this
with EACH stick in the first bank and No issue with any of the sticks of
memory. So then I doubled the test - two sticks in banks 1/3 and 2/4 - and
Vista reports Hardware Error with the memory anytime I pair the memory in ANY
bank. I don't think I have a memory issue since they test fine and XP OS
runs great.

What doesn't make sense is that I was running those 2x512's without issue
and now they error out and Vista locks up and XP gaming freezes. I am
currently running 1x1024 in the first bank without error but I want to use
all my memory or the at the very least both sticks of 1024 for 2gb total.

Any insight is appreciated.

Asus P5NSLI
Evga 7900GT
A-Data Vitesta Memory (2x512 and 2x1024)
Intel E6600
2 WD 250WKS Sata 3.0gb/s non-raid
Soundblaster HD