You probably won't see much difference. Unless you are hitting swap a lot, then it won't make much difference. I have 512MBs and most of it is cache. Right now I have Mozilla, Kmail, Kppp, Gkrellm, xterm, and Konqueror running. I'm using 200MBs not counting the cache stuff. If I would close apps when I'm not using them, that would help. I let killall do the closing.
If you can get the mem cheap, go for it. I plan to go to 1GB when the prices get right. May add more later. May not make me faster though.
If you do, post the differences. I'm curious.