I play some online games, and the biggest difference made was ugrading from 1GB to 2GB of RAM instead of being last to join in I'm now one of the first, also the swap file was getting some hammering which caused slow loading of maps and things.
Making the move to 2GB, about £70 for 1GB, made a huge difference. It also helps with video rendering etc as well. So having the best CPU/Graphics etc will be let down by not having sufficient RAM.