what dey sed!
32 bit windows can only address a total of 4 gb of RAM. That's including video ram, so if you have a videocard that has 512 MB mem, you'll only be able to use 3.5 GB of system memory. 512 MB falls outside of the PAE (since it was given to the video mem). Having more than 4 GB of memory is a waste if you're only going to use a 32 bit os.
with 64 bit, you can access up to 128 GB of RAM, so you can go hog wild with the upgrades and your system will be able to use it.
Software support is another thing, though. If your software or game is just 32 bit, it will only have acccess to 4 gb even if you're running a 64 bit OS. But this doesn't make a 64 bit or >6GB RAM totally useless because you'll still have more mem for background tasks, which allows you to open more programs concurrently. If you do a lot of video/audio encoding or if you unzip large files on a regular basis, having more than 4 GB is awesome.
Also, the 64 bit thing isn't just for the RAM stuff. Software can be optimized for 64 bit processors by taking advantage of the extra bandwidth and sending larger/longer strings of cpu instructions (similar to how a wider road can make traffic smoother/faster, due to being able to accomodate more vehicles at the same time, as well as giving access to ridiculously large trucks). Dolphin (a gamecube/wii emulator) is a good example, the performance difference between its 32 bit and 64 bit versions can be as huge as 15 FPS.
There's also the bit about a number of malware (particularly older ones) being written for 32 bit systems, and will not be able to compromise 64 bit programs. The only disadvantage to a 64 bit system is that there are a handful of REALLY old software that won't work or will not work right, even after using w7's compatibility mode (mostly the old softwares that worked around existing OS or hardware bugs - like some of the really ancient Lotus spreadsheet programs).
TL DR version: HONG CHONG BONG ME AM CHINA CANNOT SPIKI ENLIGHS WEL.
