Memory will enable to do the things you do on your computer faster. The cpu and video card are also important if you want to do video and audio editing. Basically all 3 of these things are essential, but most of all the memory, then the cpu, in that order. But if you buy a branded comp. they'll try to bum you off memory by giving you a faster cpu. That's ok, as long as you upgrade your memory after the fact. Or, alternatively, just get one of these:
