Ram is a cheap way of getting a decent performance boost. It doesn't give you a massive frame rate boost, but it can help reduce loading times, reduces lag and makes your whole system run a lot smoother.
I have 512MB of ram running on my Windows 2000 machine at work, if I have a couple of apps open (Goldmine (a type of CRM programme), Word, Excel, Sage, and a web page) trying to swap between each one you notice the computer is slow to bring a up the page. Even if I'm just surfing the net if I have a couple of tabs open in Firefox selecting each tab can take a couple of seconds. And that’s operating on a Windows 2000 machine, I hate to think how bad it is on an XP machine with only 512Mb of ram.
You’ve got a decent spec already, and 2Gb of DDR2 isn’t a lot of money. If you are going to get some ram I suggest you use the Crucial memory advisor tool on the Crucial website to help you find what ram you need.