The DDR3 memory clips are very affordable these days. The best is to buy some extra memory.
Agree that you should use the Task Manager to check on the actual memory usage and see how much are in use/available/etc..
Note 2 things:
1) Windows 7 by default has the aero peek turned on. (e.g. fanny alt-tab to switch between different apps, transluscent title bar, etc.). Those tend to eat up system resources (CPU cycles and memory). You may want to turn them off.
2) Many box makers would stuff the desktop computers that they sell with not-so-useful apps (e.g. trial anti-virus, media control, etc.). You may want to un-install those apps that you don't need.