Your main problem is probably lack of ram, though that video card is not helping matters. Upgrade your machines to at least a gig and your performance issues should start to disappear. You also want to upgrade to a geforce 7xxx series card of some sort whenever you have funds available.
Another thing that would help would be going to gigabit ethernet. The new linksys wireless N router (the silver one, not the blue one) has a gigabit switch, that's 10/100/1000 capable. if your motherboards have gigabit cards already, well worht the upgrade cost in my oppinion.