If you added extra RAM, Your computer may not support that extra RAM. You could also be using a 32bit version of Windows which would only allow 4GB of RAM to be used. Even if your PC diden't support the extra RAM it may still be working with a limited amount. Also: I believe Java errors if it can't allocate an amount of RAM anyway.
Lets do some tests
Click the Start button
, right-click Computer, and then click Properties.
If "64-bit Operating System" is listed next to System type, you’re running the 64-bit version of Windows Vista or Windows 7.
If "32-bit Operating System" is listed next to System type, you’re running the 32-bit version of Windows Vista or Windows 7.
Let us know which one it is.
In the same screen. There should be something that said what Processor and how much RAM you have. Give us that as well.