I agree with everything that both Assman and frenchsquared said. I disagree with one thing you said you were going to do (and asked for comments)...
And i'm going to add 512 more MB RAM.
What do you say?
I think you should only
add one other 256MB ddr2 dimm to just match the other 3 that you already have. And right now, you should be able to pick it up pretty cheap. You may not be able to get an exact match to your other ones but anything as fast or faster than "256MB (PC2-4300)" should really help your system...This one from Newegg is only $18 and you should be able to feel the difference.
Whatever you do with your memory, from now on always do it in pairs (i.e. buy 2 x 512M or 2 x 1G).