I have a problem with someone's else PC. He have an ASUS K8N motherboard, 1536Mb DDR400 (3 DIMM), AMD Athlon 64 3500+ CPU... He find his computer to be slow. So he tested it with a benchmark program called TestLab.
Here's the pic of the results:
512Mb = 115 score
1024 = 97
1536 = 42.
So he was right, what's the problem you think? It's all DDR400 memory but not the same brand. When he test them each one individually, he get the same 115 score. It's when he begin to add more RAM that it slow down. He updated to the latest BIOS (1010) and ti didn't helped.
I'm not sure about the specific motherboard you mentioned or just how much of a noticable change there is when adding more dimms, but typically you will notice a change when using more than two slots for memory, just as double sided and single sided dimms can make a difference in performance, using too many slots can diminish memory performance. That's one <small> reason why larger single sided memory carries a price premium. However it should not be THAT noticable, if it really slows down performance there may be another issue at hand.
EDIT: You may want to verify whether dual channel is running on that board... that could also make a difference.
I don't know that program, so I'm going to take those numbers at face value. I think the problem is the motherboard lowers RAM speed when more than 2 sticks are installed. After all 3 sticks are installed, go into BIOS and make sure the RAM speed is set to 200, not 166 (or 400 not 333 depending on how it's displayed).
With 2 sticks in the right slots 1gig should work in dual channel (assuming that is a dual channel mobo and I think it is but I didn't check) - mismatched memory may void that - and I would think it should give at least as good of a score as the 512meg one. With 3 sticks it will not be able to work in dual channel, so scores will be lower (but I don't think it should be any lower than with a single stick).