I presume you have a VERY tight budget since RAM is so cheap these days...
If you have 2GB (2x 1GB sticks) and have room for two more sticks - a pair of 2GB or a single 4GB for a total of 6GB would be a BIG improvement.
(let me know what you have, I'll tell you where to move stuff for the best Intel memory channeling performance... every bit of speed counts!)
If you have one 2GB stick and 1 or 3 empty slots, I'd add a single 4GB stick for the same 6GB total.
Two 1GB sticks but NO empty slots? Pull one of those 1GB sticks out and replace it with a single 4GB stick for a total of 5GB.
2GB of RAM is simply insufficient for all but very basic use. You plan on gaming... you need more than 2GB.