Originally Posted by tcracker
Most people the 32bit XP. In that case, the max will be 3.5GB
This is incorrect. A 32bit OS has the ability to address 4 gigabytes of memory. Now you may notice after you dropped in your 4 gigs of ram it shows up and 3.25gb or 3.5gb, this is because there is other memory in your computer that is address before your RAM. This could include a video card, sound card, or anything else in your system, the RAM is the last thing that is address and gets whatever is left over.
tcracker's other statement is correct - 64 bit OS can address much more, some versions of vista such as ultimate can address 128gb I believe.A 64 bit OS can address up to 2^64 - 1 or ~18.45 exabytes, more than we could ever comprehend using currently.