Soldato
Got myself a GA-P31-DS3L yesterday..
Apparantly this problem is due to a "chipset limitation" in the P31, it will only run 4x "single sided" modules and not the double sided type which most memory is today.
I have the same issue, and I've had 2GB on my desk for a month now just waiting to be used..
tbh, the P31-S3L is a budget board, and although it clocks okay, I'm now off to buy a new board, rather than new ram to fit the board. I'd advise you do the same.. it'd make much more sense to upgrade the board, than try finding 4GB that works