Unreadable Ram

I added RAM (2 x 256MB) to total 1GB on my computer and ran the DirectX Diagnostic Tool to see if it picked up the new RAM, but it still says I have only 510MB. Do I need something to get it to show, or does DxDiag not read newly installed RAM?

Several things might be going on here. Some motherboards don't play well if you add memory to all the memory sockets. You need to look in your motherboard manual and check to see how many "banks" of memory it supports. Memory modules with chips on one side have one bank. If a module has chips on both sites, that usually means it has two banks. Some motherboards support a total of only six banks of memory, or even less.
So if your existing memory already used up four banks and you installed two more double-banked modules, that memory may not be visible. Or it may simply run more slowly.
Of course, there's always the possibility that your new modules are defective. You might try pulling your old modules out to see if the new ones actually work.