Okay, slow down.
Before upgrading your memory beyond 4 GB, see how much it is actually using. Probably the easiest way to find that out is to press Ctrl+Alt+Del and start the task manager. Go to the Performance tab and see how much physical memory is actually being used. If the amount in use is > 3.5 GB, then a memory upgrade might very well help.
If the memory isn't running up against your 4GB configuration, adding extra isn't going to improve your performance at all. Slowness is more likely due to background programs using up computer resources (search indexers, chat clients, etc). Staples can help with that, or you can do it yourself by removing any unuseful programs you have currently installed (check the control panel).
Upgrading to 8 GB will require dumping your existing 4GB of RAM and installing two new modules. That's bad news for your PC, because 2x 4GB DDR2 modules will run you $250 or more. Staples.com doesn't list that type of memory for sale.
Taking advantage of their PC Tumeup services would be your best bet to speed things up without breaking the bank.
EDIT - based upon your Sims 3 entry:
That laptop has integrated graphics that simply aren't up to gaming standards. Being a laptop, the graphics card can't be upgraded. If you want to game, you'll need a new PC.