Not sure on this, but I know having 4x512 makes you take a hit on memory dividers and instead of 1Timing it's 2Timing. The divider is not on the venice chips. I'd dig deeper in the forums. I even think I asked the same question!
Yeah, I've got the same board and stuck with my 1G of ram for now! I don't want the performance hit and don't have the money to upgrade.
-JC