Definitely RAM. Buy the 2 sticks of 1GB and sell your sticks if you can. Otherwise, use them in another system you could build for random tasks or just hosting files for a home network. Whatever the case, the RAM is the best way to go with what you currently have.
I wouldn't put 2 of the 256MB sticks in by the way. Straight up 2GB dual channel, sell or use the other sticks. Then afterwards (of course), get a nice 256MB video card and you'll be set.
Case: LIAN LI PC-A70B
Mobo: Gigabyte P35-DS3R
CPU: Intel Q6600 g0 @ 3.0GHz
HSF: Thermalright Ultra-120 Extreme
RAM: 4GB G.SKILL @ 4-4-4-12
VCard: EVGA 8800GTS G92 @ 700/1000
HDDs: 2x 750GB & 250GB
Optical: Sony NEC SATA DVD Burner
PSU: Silverstone 750W Modular