Ive ran many benches with 1T vs 2T. Unless youre a hardcore 3D bencher, youre not going to see a bit of difference. SuperPI times drop by a whole second, which is nothing to be concerned about. Memory bandwidth does not drop in sandra or any of benches. Until you run 3Dmark, thats where you see a drop off.. With the amount of ram these new games need, that 1 gig will make up for the non-noticeable performance hit you see on some benches.
If you want to keep your high overclock, what you need to do is this...
If you have high performing ram now, try to get another 2 X 512 package. Try to keep the same ram, but you can mix and match manf, just keep the UTT/BH5/VX,Micron chips together.
Run each stick at the Dimm slot closest to your cpu... Then load up mem test.. See how far each SINGLE stick can go... Take the highest clocking stick and place it in the further dimm slot from the cpu. Take the weakest clocking stick and place it closest to the cpu.. What you will end up with is a well balanced , high clocking setup.. I can manage 265 fsb with all four sticks enabled. Sure, its at 2T but you would never know the difference unless you benched in 3dMark..