Indeed, DDR3 is quite affordable, and while I'm not sure if going from 4 to 8 gigs has a decent impact on current games, it's smart to save two slots on the mobo for that upgrade, if you can.
Mind you, ram is a component where you don't want to go for cheap - get a decent brand and double-check your motherboard and cpu ram compatibility tables on their vendor's sites. It might seem excessive work but it could save you headaches and a RMA.
You're welcome. :)