Windows XP really needs 512+MB to begin to run smoothly; at 256MB ram, you've barely got enough to start Windows without it hitting the HD pretty hard. With RAM prices *dirt cheap*, you can usually get 2GB of RAM for $15-30. (Granted, if you've got a oddball memory format, it can be more expensive. And if your MB choice is anything like your choice of modder's tools, BNG, you're very likely to have an oddball memory format.)
I'd go to
http://www.crucial.com/ or
http://www.kingston.com/ to figure out what type of memory your system takes. Then, take their part #s, and go to
http://www.newegg.com and get the same thing for a little less. I don't go for the cheapest RAM makers, as they tend to flake out, but Kingston's been quite reliable for me. My rule of thumb: if you can max out your ram for $30 or so,
do it. It'll make everything faster.
-- GSH