But, as said, two sticks may enhance performance (though it never has for me... weird...) but two 2GB sticks are rarely cheaper than one 4GB one (also if he ever switches his CPU out for a better one and his GPU for a better one, he'll probably want to upgrade to more than 8GB, so he'd have to sell all the sticks and get new ones)...
And believe me, I run a very similar system to the OP's one. 4GB of RAM, and it rarely goes over 3.5 (Win 7 Aero theme on, plus other eye-candy and background programs/processes). :/
Also, @OP, watch the voltage spec levels (never over 1.5V) and latency. I recommend Crucial or Kingston, though Corsair ones are said to be excellent as well.