I agree with Lmeow.
The speed of the ram has a minor impact on performance, compared to the gain of added ram.
If you can find an extra 2gb kit exactly like your current ram, then it should work.
But to be certain, market your current ram on e-bay and replace it with a matched 2 x 2gb kit.
Go to the ram vendor's web site, and access their configurator.
Corsair, Kingston, Patriot, OCZ and others have them.
Their compatibility list is more current than the motherboard vendor's QVL lists which rarely get updated.
Enter your mobo or PC, and get a list of compatible ram sticks.
Here are a few links:http://www.crucial.com/index.aspx http://www.corsair.com/configurator/default.aspx http://kingston.com/ http://conf.ocztechnology.com/index.php?c=1 http://www.patriotmemory.com/configurator/index.jsp