You don't mention the rest of your rig, but if you now have a dual channel memory configuratrion, you lose it if you go to 3gb, and windows stumbles at 4gb. Even if you just run new ram at a higher speed, you may have a performance hit by running the ram and the cpu at different speeds. You might get an improvement by overclocking with more capable ram, but then there are other issues to contend with then too.
Bottom line: don't spend the money on ram, but a better vid card (if gaming) or a faster cpu instead (assuming the mb supports it).