From 9to5Toys.com:

If you haven’t already upgraded your recent iMac, Mac Mini, or MacBook/Pro to 8GB of RAM, there are only 44 little dollars between you and a much faster Mac that can run more applications at one time.  9to5 affiliate partner Amazon has the Corsair 1333MHz Unbuffered CL 9 SODIMM Memory 9-9-9-24 1.5V DDR3 8GB Memory kit for $53.99 with a $10 rebate that runs out this weekend.

If you’ve got an older Mac that takes 1066MHz RAM, you can save another two bucks.

Upgrading the RAM in you Mac is usually pretty easy but see your Mac’s documentation for specifics.  Here are a few video guides:

Apple usually charges $200 to upgrade, so at $50 for 8GB of RAM, there is no reason not to get more new RAM. Hook yourself up with a 1TB hard drive ($109) or $256GB SSD while you are in there.