Shoutbox

I have a 2nd PC with this MOBO:

http://www.msicomputer.com/product/p_spec.asp?model=PM8M2-V&class=mb

I currently have 512MB of RAM installed, but would like to upgrade the RAM to say 1Gb total (is that the bare minimum for increasing performance?)
I want to spend the least money on this PC since it's an oldie, but recognize that the current 512MB makes it slow (plus too much hard drive activity).
I'm confused as to the types of RAM modules, eventhough the MOBO specs detail the compatible RAM types.
So how should I order the cheapest 512MB module (or maybe use a single 1GB?) for this MOBO? DIMM or DDR? If DDR what type? 333 or 400?
Thanks.