Have just taken out and had a look at my ram....it's kingston valueram KVR400x64c3a/512
this page tells you how to decipher all that.....
ValueRAM - Desktop Memory - Desktop Memory
which tells me that I've got 512MB 400MHz DDR Non-ECC CL3 ram. (which still means pretty much nothing to me, other than it's nor DDR2), oh and it's PC3200
So, I'm guessing that I could upgrade to 1 GB by buying another 512MB ram that is exactly the same and sticking it in the second slot. (or would replacing the 512 with a new 1GB be better???

)
I've got 4 slots, so do you think I can keep adding up to 2GB???