Hmmm.... I'm running a PIII 700 with 192mb RAM and I can have several windows open, Zone Alarm, Norton, with a game running in another window, although no email program. I am not having any trouble doing this.
I'm also using PNY memory in my gaming computer and it's been perfect. Gosh, I feel like a rabble-rouser on this thread. :-)
You can use more "memory" than you have physically installed, it's called virtual memory. When the operating system needs more memory than physically available it swaps physical memory data to the hard drive to free it up. That's all fine and good except it is slow. Real slow... And less reliable.