Geek post ahead!
A 32 bit OS can only address a total of 4 gig of address space. It’s binary, which means 2 positions (1 or 0 – On or Off), so 2 to the 32nd power:
2^32 = 4,294,967,296 or 4 gig
However, the OS must reserve some of that space to address internal hardware like cache, USB & PCI interfaces, etc. Depending on the system and BIOS, this can take 500 meg, and up to about 800 meg.
So realistically, all the ram a 32 bit system can address is 3.5 at best, but typically, 3.2. You can load a 32 bit system with 4 gig (it’s actually cheaper than using 1 gig modules) and it won’t hurt a thing, but you won’t be able to use that upper 500 – 800 meg. Your system will only report 3.2 to 3.5 of ram installed.
A 64 bit OS can address 2^64 = 18,446,744,073,709,551,616
That’s 18 Exabytes. An insane number.
The naming is: Exabytes, Petabytes, Terabytes, Gigabytes, Megabytes, Kilobytes, Bytes
The only problem I have with a 64 Bit OS is that many popular programs are 32 bit and aren’t guaranteed to work with them, so there is a compatibility issue, especially if you’re doing video capture and high end graphics work. Plus, 64 bits for Windows means Vista right now, and it’s just not stable enough for me. Windows 7 is on the way, but it’s fresh out of the blocks, so it’s sure to have stability issues until SP1 or SP2.