Dan,
It sounds like your motherboard cannot support more than 256MB of RAM. The fact that you have the latest BIOS is great, but that cannot eliminate hardware incompatibilities. For example, I have a motherboard with two slots, filled with 512MB SDRAM DIMMs. Even though there are now 1MB SDRAM DIMMs on the market, I cannot use them, because the hardware configuration cannot address 1MB DIMMs, period.
To try to explain, without getting overly technical: Each DIMM module has a number of signals coming to it to address the individual bytes of data in a row/column method. Each of these signals if connected to hardware in your chipset that keeps track of the memory being used, and the next location to be accessed. Depending on the memory used, the external signals may be used for different purposes. If I had to venture a guess, your PC has enough external signals/chipset hardware to address a 256MB DIMM, but only enough internal/external hardware to support 256MB of memory. This is why you can use 2 x 128MB or 1 x 256MB, but 2 x 256MB will not work. In a perfect world, the BIOS would compensate for this -- or at least, tell you that the memory configuration was wrong -- but it's difficult to support many different sizes of memory and detect wrong combinations, too.
If you really want 512MB of RAM, you need a new motherboard. :^(
Regards,
-BW
--
Bruce Wahler
Ashby Solutions™
http://music.ashbysolutions.comCloneWheel Support Group moderator
978.386.7389 voice/fax
bruce@...