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The problem is that with that board is when you use all the ram slots the speed of the memory gets dropped to 266DDR.
Add another 512 to make 1024 but only using 2 slots, the speed of the ram will stay at 333.
But with your 333 on your 2100 could be a little bottle neck on your 333 memory there. You wouldnt notice a difference only on very demanding apps.