download and install Everest Home, while running your normal set of tasks, open Everest and look under "motherboard" then "memory" see what your Physical memory utilization is, if its up in the 75% range you could probably benifit from the ram upgrade
oh, and just buy a 512 stick to add to your existing, however if your mobo/cpu supports a faster speed than you have then you may want to consider getting a gig or 2x512 of the faster ram, ie if your mobo/cpu is 333fsb you will want at least PC2700 DDR333, if 400fsb, you will want PC3200 DDR400, etc.
Also, dont know what you have now?, again Everst Home, "motherbord" "SPD", under memory speed