Not booting after added RAM is a common issue. Try putting one stick of RAM in at a time to try and determine if there is an issue with one of the sticks or slots. Are you getting anything on the screen with the new RAM in it?
As far as an fdisk prompt. Don\'t play around with that. It "reset" your computer\'s hard drive forcing a reinstall of windows and losing everything. In windows try doing a disk check to see if there are any errors on your hard drive.
What kind or error are you getting when you try to burn a cd? Memory speed should not be an issue as the new memory isn\'t really all the faster then the old stuff, there is just more of it.