it really depends on your budget. For a big performance boost, u can upgrade to 2 gigs by buying 2 sticks of OCZ, Kingston, or any other high quality ram.
If you want to spend less than 200 dollars you could get 2 sticks of 512.
Both of these options will give better performance than your current setup.
It might also be a software issue. Prebuilt computers come with a crap load of software u don't need. Unless you want to watse your time uninstalling or disabling some of them, I say just install a fresh windows XP in there and SP2.
edit:in order for dual channel, u must have 2 pairs of matching ram. If you buy something like this: http://www.zipzoomfly.com/jsp/Produc...ductCode=85209
, the two sticks will be matching.
The link I posted is actually a pretty good deal.