The Max memory is limited by the chipsets on the laptop/ Desktop motherboard not the OS.
Windows 7 x64 can use upto 192 Gig. The laptop can only handle 4 Gig.
Most motherboards have a memory capacity of 8 to 24 Gig. Servers can handle alot more RAM, up to 256 Gig. Most laptops max out a 4 Gig.
Most laptops have 2 memory slots, Most slots on laptops max out on 2 Gig per slot. some max out at 8 Gig.
Most desktop boards with 4 slots max out at 8 Gig
Most desktop boards with 6 slot max out at 16 Gig or 24 Gig.
Plus Some chipsets have limitations on the amount of RAM that they can support. The ASUS CUSL2 motherboard had the Intel 815 chipset that could only support 512Meg of RAM, So they only put 2 slots on the motherboard.