Just read this, the bit that has got me on the compatible memory page:

"This system requires single-sided/single-ranked memory modules when all four sockets are populated.
1GB double-sided(dual-ranked) memory modules may be installed one per channel. For Dual Channel mode, use
single-sided modules only."

Which to me sounds like its limited to 4gb. So if you are using ALL 4 dimm you can only use 1gb per slot. I still don't see why you can't use 2x2gb sticks. However, they would still have to be SS due to the dual channel restrictions.

Anyway,

I think the reason ram is not listed as single sided and double sided any more is because its very rare that a mobo can only read SS. So I now see why your are struggling to find some.

You may be better of upgrading your mobo as you are really going to struggle to find SS ram that is also the correct speed etc...