first, most important is that we have to know if your motherboard supports pc3200. If it can't , you're stuck with what you got. or get another pc 2300 mem stick
If your mobo does support pc3200, then your best bet is, as blind_arrow already mentioned, sell both what you got and buy 2 x 512 pc 3200s. If two different speed mem sticks are installed, the higher speed one goes down to the lower speed. So it's just a waste of money to get a 3200 if it's just going to end up being a pc2300.
installing two different speed mem sticks shouldn't be much of a problem unless it has compatibility issues. You should see some significant speed changes in games.
hope this helps??