RAM suggestion

Getting another 4 GB, which means total 8 GB, will only benefit if you have heavy memory-consuming routine, such as running virtual machines servers, doing heavy AutoCAD and 3DMax at high resolution rendering, running heavy, maybe 5 games at the same time (like Arkham Asylum, Mass Effect 2, Starcraft 2, Metro 2033, and Crysis at the same time).
8 GB is very good space, but it won't give faster performance than 4 GB. It just gives your computer more memory to store larger data, but doesn't heed performance gain.

..It kinda works parallelly, not serially, with computer performance. The more RAM, it only means just as far as not too much "retrieving data from harddisk", but does not heed additional transfer rate. Why, because it's capped at processor's bottleneck, and harddisk transfer data. So if you're running applications that has requirement of 4 GB is more than sufficient with, adding to 8 GB will not yield any performance gain.

...if you're average gamer which runs only one game at a time, 4 GB is enough, at least for now. Should expansion requires more, you can add it later.