Memory for a P5AD2-E

I've currently got 4 x 512 (2 matched pairs) of Corsair XMS (TWIN2X1024-4300C3) memory on an Asus P5AD2-E. I was thinking of replacing that memory with 2 x 1GB memory and freeing up the two matched pairs for use in two folding systems using a reference Intel board I've been unable to sell and another Asus board currently in use. I want to keep the P5AD2-E in use a bit longer and the memory in that system is the only memory I've got left that's under 1GB--I could split the pairs up and still have dual channel memory for the folding machines.

So, my question is: what would be the best memory to buy for the P5AD2-E? How fast could I go? And, please, nothing "top of the line". This is a two year old motherboard... I'm just trying to get another year or two of use out of it.

Thanks!
MVC

Get something cheap as long it keeps 1:1 ratio.