Okay, so I'm running a 5850 HD. This isn't a dedicated mining rig, it's a gaming computer that I just happen to use for mining as well. I used to have 2 x 2 GB RAM sticks, but today I upgraded to 4 x 2 GB. Surprisingly, my hash rate went from 272 Mhash/s to 292 Mhash/s. I don't see how RAM would affect my mining, but apparently it does.
So I did some research. My RAM seems to be running in unganged mode, meaning each core of the processor gets assigned one RAM stick each. In ganged mode, all RAM is shared with all cores. It makes some sense that this would improve my processor's performance, since I have a six-core processor, and it would imply faster access to stored data. However, I'm not using my processor for mining, I'm using my GPU... so I don't see how increased CPU-performance would improve my mining.
So does anyone have any idea why my hashing speed went up by 20 Mhash/s after a RAM upgrade? I can provide more information if necessary.