Have you checked to see if the page file (virtual memory) has changed since you installed the new RAM?
To check it:
1. Open System in Control Panel.
2. On the Advanced tab, under Performance, click Settings.
3. On the Advanced tab, under Virtual memory, click Change.
If you do this you will be able to see the Minimum allowed, Recommended, and Currently allocated. If the Currently allocated is lower than the Recommended you can change it to match the recommended. If you are not getting the "low virtual memory" message you may not want to change it. The computer will read from RAM faster than it will from the hard drive.