I have a small geode board that has a ram limit of 128MB. This is stated in the doc's, I have not yet been able to test this, yet. If it does cap out at 128mb would this be a factor of the bios or the hardware, or both?
In general is a hardware limit. The ram interface is going to be designed with a specific range of sdram chip density and bus loading in mind. At a given density you have to add more chips to the bus to get more ram. This will increase the bus loading. The limit should be set for the maximim loading of the highest density at the maximum speed.
If you purchase high quality ram modules that don't load the bus as much and have very good matching on pcb you *might* get away with adding more than the said maximum. Also If you slow the bus down you might be able to push the specs. Over clockers do the inverse of this all the time. Getting good quality ram (keeping it good and cold) and upping the clock rate beyond rated specs.
Density wise you can't normally do a whole lot. Usually that needs a new sdram controller.
As with all things in semi-conductors you can probally push the specs beyond whats on the datasheet but it may not work across all boards.
-- Richard A. Smith