@amigadave

To use more than 128MB RAM you need to change the way the RAM is routed to the SoC; the board is prepped to take 128MB in 2 chips.

Adding 4 chips and increasing the RAM size means changing some of the lane such that instead of 16-bit per chip, the RAM is routed 8-bit per chip, to make a 32-bit DDR bus.

You'd need a new PCB with the correct layers. The solder pads on the underside are just 'for show' - they don't work unless the other layers match (just that etching the top layer is done exactly the same for any PCB that was made).

Add to that, the firmware needs modifying to set up the RAM with the new PCB and whatever new RAM chips appeared. The reason it was stuck at 128MB was a design decision to keep cost low; at the time the original was designed, ~512MB of DDR RAM was prohibitive and most of the chips in the configuration that was most efficient would have tripled the BOM cost of the board.