If you bought a MacBook Pro last fall, perhaps it's time to take your machine all the way to 16GB of RAM. Prices for a pair of 8GB DDR3 SO-DIMMs have fallen off a cliff over the past year.
In an excellent chart at Lloyd Chambers' Mac Performance Guide blog, you can see the tremendous drop in prices for 16GB (2x8) memory kits and 8GB (2x4) kits. The current MacBook Pro (2011) model can handle 16GB total and the previous rounds can take 8GB total.
There was a big drop last August in prices for 8GB DIMMs and then another drop that started in October.
Modern OSes such as Mac OS X want as much memory as you can throw at them. It's the easiest way to get some extra performance.