What limits my laptop on upgrading RAM at 8GB?

I believe the memory controller of the Intel HM55 chipset has this limit. Looks like a design fetaure/limit related to Intel's design. The exact model of your Protege 780 might result in some additional feedback from others

I won't get into detail about your specific machine, actually it seems you want a more general answer. My following answer is mostly backed by this article, read up on it if you want more specific knowledge.

This limit is caused by your motherboards hardware. A recent 64bit processor is limited to access of 64GB, this limit is a hard limit caused by the available pins on the processor. The theoretical limit would be 2^64. (But there is no current need for this much memory, so the pins are not built into the processors, yet)

The northbridge manages a so called memory map which maps areas of the memory to be read and written to by certain devices, the northbridge has a hard limit too. Remember that every pin and every connection on the motherboard makes designing it harder and the chips places on them more expensive. So this a sheer cost-factor, the manufacturer just assumes that most people will not use more than the limit given by the hardware. Hardware supporting more, is more expensive.

The Bios has nothing to do with anything on your computer as soon as your kernel is loaded. Note that recent processors embed the northbridge, I am not sure how the limit is defined on motherboards without a northbridge. (But I think the limit still is not only defined by the processor)

Kingston knows about 10 versions of protege m780 accepting from 6GB (2+4) in single channel mode to 16GB (8+8) dual-channel. Probably you need to figure out modification of "Protege M780" before approaching the vendor.

Memory supported depends on how pins are connected on motherboard to actual RAM sockets. From description of kingston i'd guess Toshiba wanted to have two 4GB options i.e 4 or 2+2, gladly they designed BIOS well and maximum module in each socket works just fine.

I'd guess actual CPU might go minimally with 36-bit addressing i.e 64GB, mine (Acer, Athlon 64 of 2006) for example could handle 40bit physical addresses - 1TB, but motherboard has two slots where DMI says each is 4GB (and motherboard has logic present to support it) but they accept in fact only 2GB modules, ignoring 4GB ones even if plugged, probably to save annoyance for XP users which max out with /3GB switch

It is merely wire not drawn from motherboard controller (aka northbridge) to memory slot, so there is no household way around it (i.e even good soldering skills will not help)