I'm pretty certain the issue is not a reg tweak but rather the virtual cache tweaks I mentioned above.
StealthyV: there are a couple of reasons that your system does not recognize the 512Mb module: 1) Your mother board was not designed to recognize anything bigger than 256MB per memory slot. 2) Ther is an issue of density on the memory module, in other words how many chips are on the module to achieve the quantity of RAM. Newer menory modules can have as few as 2 chips versus older modules that needed say 16 chips to get the value. It's kinda lengthy to explain but older systems can't recognize all of the memory available from a module if the density is to high.
You may be correct that this is a BIOS issue but that will be a new on on me. If you want post your MoBo model and also the 512 modules model # and it should be a quick thing to figure out why only 256 is recognized.