If you are going to mix and match RAM (which still isn't recommended though it's more of an old standard) you should pair the memory by size in each channel or at least that's how I did it when working on someones system. Pair the 2GB sticks in slots 1 & 3, and 1GB sticks in slots 2 & 4. Slot 1 being closest to the CPU.
Hopefully that helps!
that's seems to have done the trick - it is showing up as 6 gigs. I intend to grab another 2 sticks shortly to bring it up to 8. Will I see a benefit from 4 to 6?