Corsair RAM question/issue

I have an abit mobo with an e6600. I had 2 sticks of corsair xms pc6400 in there and everything was fine.
Today I added two more sticks (all 1G) and things worked fine until playing Battlefield 2. My box locked up-- three finger salute did not even work.

all four sticks are corsair was 5-5-5-12

The only difference I see on the sticks themselves is th first pair says version 5.1 while the new ones say 5.2.

Could this be the problem?

I ran memtest with all 4 sticks in and started getting errors (but I ran 3 tests simultaneously as directions said).
I m running a new test withjust the 2 new sticks.

Any help would be appreciated.

yes. I have winXP.
It shows 3GB RAM in the system, but my understanding was the other 1G gets dedicated to the OS.

I have a HD with Vista I can see if BF2 runs under that with 4 sticks in. If it does, I guess it must be XP. If not, could the difference in v5.1 & v5.2 be the issue or is it more likely a bad slot on the mobo? (I sure hope not--it's only a few months old). This brings up another question... can I skip slots 1&3 and use slots 2&4 to test 2 sticks in those slots?

Incidentally, the memtest ran all last night with the 2 new sticks and no prob.

I have an Abit AB9-pro
I think it is at 1.8v if I read uGuru right (1.9 is Corsair's spec)
I have not made any changes to anything