My 2 x '4GB Corsair XMS DDR3'sticks arrived today. I had used 2 x '2GB Crucial Ballistix DDR3', and had looked to upgrade them, but sadly my motherboard didn't like having another two that supported XMP (something almost all RAM sticks have), so I gave up and just decided to replace the current two with bigger memory sticks.
After installing I used a very clever tweak my friend Staeton suggested (only suitable if you have a lot of RAM, 8GB+, and a hard drive that's quite slow).
You can significantly decrease the size of Virtual Memory or completely disable it. Virtual Memory, is effectively fake RAM on the Hard Drive. So if you have at least 4GB chances are, when you switch between several open programs, the giant lags are due to a slow hard drive rather than 'not enough' RAM.
Virtual Memory is stored on a file called 'pagefile.sys'. It's a hidden file in your C drive (or whatever drive you're using for your Operating System). You can easily see it by altering folder viewing options: Uncheck 'Hide protected operating system files (Recommended)'.
My 'pagefile' took up a crazy 8GB, it was almost like having crappy RAM.
Here's where you find the settings for virtual memory
[Warning: if your computer is not suffering from lag and you have a fast hard drive or a Solid State and decent RAM, then FABULOUS, you shouldn't have to do this. Also do not do this, if you have no idea what you are doing- I use Windows 7 x64, and my specs are found at the bottom of this post]: