Well, while they say that there are no stupid questions, there are certainly some questions, which, if answered directly, do nothing to help the OP.
Looking at the website of my local dealer, the largest memory modules that they stock are 4G, and these cost £26 each. For most consumer boards, with a single memory space, you’ll have a maximum of 4 slots. So, even doubling those sizes, the maximum memory that you could plug in would be 16 G. And, even at a future time at which 16G modules become available, 64G would be the maximum practicable in a board with four slots.
The architectural limit imposed by the CPU is 48 bits for adressing which is
256 TB. This is as far as I remember not usable due to several constraints
which limit it to as far as I remember 16 TB. But I am also now to lazy to
look up the proper links which give an authoritative answer.
The limit imposed by the hardware can be one of several numbers, depending on which exact hardware we are talking about. If we dismiss older hardware (there are relatively recent chipsets with a 768M limit, which is a bit shocking) then we could be talking about processor limits of 36 bits (probably the P4 is the only example of this still in common use), 40 bits or 48 bits, although, in practice, chipset limits may well be lower.
If 32 bits worth of memory address space costs you £26 to fill, then 36 costs £406, 40 bits £6496 (512 sockets on your motherboard?). This is money, and it is left as an exercise for the interested reader to work out how deeply in debt you would be at 48 bits of address space, even if you could use it. I’m too indolent to be even interested. Or get my pocket calculator out; I take no responsibility if I’ve made an error in calculating those figures, but, rest assured, they get big fast.
The memory limits for other Archs, such as Itanium, Power, ARM, MIPS and PA-Risc variants will, of course, be different from those of x86.