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.