umm disreguard most of what you have already been told in this thread.
Assuming that someone has 4 dimms on there motherboard, if even they are dimms, is a poor attempt. I have 3 comptures running in this room, one has 1 dimm, one has 4, and one has 3. Both of my machines at work have 3 dimms.
Also, you dont just buy a 2gig stick of ram. Thats not how it works. You have to find out how much memory one bank can actually handle, or it wont work at all.
Assuming that more than 2 gigs of ram will slow you down is wrong also. Its not page filing/virtual memory.
If you are hungry for information on how memory works, read up on terms like
DDR, DIMM, SIMM, RIMM, SDRAM, DRAM, SRAM, and how it all works. To really get into it, you need to know the system bus speeds, or FSB(Front side bus), and what your chipset is capable of, etc..