Ditto to aliasaid,
In addition another 512MB (2 x 512MB) in Dual-Channel mode will yield faster transfer rates, just make sure you use the appropriate slots. For the most part ATX boards will have 4, 184-pin DIMM slots and Mirco-ATX boards will have only 2. Discern which one you have.
Remember, try to add memory with the same timings. You can verify your timings with CPUz, Everest or SIW. An online search will direct you to the appropriate sites.
[SIZE=\"1\"]E6400 @ 3.4 , Noctua 120
7900GT @ 670/1680 , VF900
2 x 1GB Patriot 6400 @ 425
WD SATA 250GB
Neo HE 550
P180 / G15 / MX1000[/SIZE]