How do you figure the MBR is corrupt?
I remember reading somewhere that it can cause a prolonged blank screen during startup.
Depends on where you are seeing it...however a corrupt MBR wouldn't cause slowdowns during anything but boot.
Get rid of the freakin 256mb DIMM. Memory can only run at one frequency (unless you have dual channel and then its one frequency per channel). Guess whether it runs at the slower or faster frequency.
Defrag the drive.
If it is an ntfs formatted partition, run chkdsk on it too. Then run it again unless it said it found no errors/didn't fix anything. These should be obvious things before upgrading memory. I'm obviously too late or I would have recommended you use Mushkin's memory finder instead of Best Buy as well.