RAM RAM RAM RAM RAM!!?

That's normal for 32-bit versions of Windows desktop operating systems. You can lose anywhere from 256MB to 1GB because of the the need to reserve address space for memory-mapped I/O devices. A 64-bit version of Windows won't have this problem, or a 32-bit version of Windows that supports PAE (physical address extension)

When you start the computer, the BIOS should correctly detect all 4GB or RAM.

this is probably because you have a 1GiB video card installed. it uses up 1GiB out of the 4GiB of RAM you have, leaving you with 3GiB of RAM.

very likely you also have a 32-bit system like mine due to the 4GiB ceiling. you didn't mention the mfr, that would help, there are about 300 of them I guess.

the 1GiB video card will make 1GiB of RAM disappear if the system has a 4GiB ceiling (32-bit).

the amount of RAM a 32-bit system supports is always 4GiB (2^32 = 4,294,967,296 = 4GiB). then there's the amount of RAM your mobo supports, that's a different ceiliing. subtract from that the amount of VRAM your graphics card has, and this is what's left.

on a 64-bit system (or a system with address extensions, same thing), the 64 bits really only applies to the data bus, not the address bus, since the address bus on today's procs cannot address the full 64 bits. also, on 64-bit systems, I suspect that video card memory (VRAM) doesn't take up the same address space as RAM since there is a LOT more address space, on the order of anywhere from 16GB to 256GB for windows and most intel processors.

YES YOU CAN MIX IT!!! ram amount is depend on your motherboard... if your motherboard only can support until 2gb of RAM thats mean you cannot use 3gb RAM or more Find RAM? try crosair, its the best!

If you have 32 bit Wndows it can only use 3.5 GB if you are very lucky. Most often only 3 GB. So that is all you get.