1) Updates are a good thing, as Beefy mentioned.
I had a system acting the exact same way the other day- drove me nuts! I tried everything until finally I loaded bios defaults on the system. This finally fixed it. SO it's worth a try i guess. ;-)
2) Are you sure the MTB you have will accept the new style 512 memory sticks out there? The pc133 memory that is most prevalent today have a higher density type of chip on them. They are usually 8 chips/side. Older MTB's have a hard time recognizing the new density chips. If your MTB is older than 8 to 12 months there is a good possibility it will not work with this type of ram.
Old MTB & 512 sticks = either double stacked chips, or double rowed/side!
Old MTB & 256 = 8 chips /side
OLD MTB& 128 = 8 chips one side - one side only!
New Density 512 = 8 chips / side
New Density 256 = 8 chips on one side only, or 4 chips each side
New Density 128 = 4 chips one side
hope this helps. :-)