Oh, it's an A7N8X, is it? I had one of those, and it was a bitch to find memory it liked. Asus boards are often very fussy, and very often need identical sticks of ram ( not a matched pair - that's for performance junkies, not compatibility - just two identical sticks. ) By all means just buy the 512, but if it doesn't work or if you start getting out of memory errors and crashes, you'll have to pull the 2x256's back out and get another identical stick of 512.