You should be able to add more RAM. If for some readon you only have 2 DIMM slots and they are occupied by 2 x 128MB sticks you will have to replace one or both.
Make sure that you get the appropriate speed. PC3200 (DDR400) is most common today. Kingston, Corsair, OCZ and Patriot are all reputable manufacturers with Value RAM series.
You can purchase them online at:
Zip Zoom fly
Or at a local retailer, though online would probably save some cash.
[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]