Let me start out with repeating what someone said in a thread on these forums awhile back when I was just getting a second 512 to run dual channel
"Damn guys its not DUEL channel its DUAL.... The sticks of ram arent fighting each other!" lol.. Just thought Id let you in on that.
If youd like to test the bit of performance diffrence your getting from your ram, test the read, write and average latency using either Everest Home or SandraSoft. Everest Home is free and so is SandraSoft if I remember right.
With everest home dont run anything you wouldnt normally run in the background at idle, dont even move the mouse.. Itll pause for like 5-10 seconds and then give you a read out of the Read or Write or Latency depending on which you told it to benchmark. And it will compare it to other machines using whichever speed of ram, mobo, chipset, and CPU.
Then simply fidle around with diffrent sticks of ram in there to see how it runs speed wise to size-wise.
One thing that bugs me is how people who upgrade from 1gig to 2gigs expect to see a speed diffrence lol. 512-1gig possible.. 256-512 HELL YES (With XP especially). I guess people consider the size of ram they have as more speed, not true unless you have applications that even use that amount of ram. 1gig for most gaming will be fine, if you want to somewhat future proof yourself id go for 2gigs dual channel 2x1gig DIMMs. If your not using many applications that use say 1.5gigs of ram and you have it in there thinking its going to speed your rig up, your better off using just 1gig if it runs faster.
So give that a shot and see what configuration of all your extra DIMMS come out to. change it around and test til you find something to your liking. Id recommend 2x512 and no more unless your shooting for programs that can use more than 1gig in which case for with the 2gigs dualed