A couple of minor things to add, though nothing as impressive as Jofm5.
The memory address is maximum 4 Gb total, I think it's actually a little less, but the total includes graphics card, cache etc. so if you have two 512Mb graphics cards, and 4 Gb RAM, only 3 Gb of your RAM can be addressed.
Apparently 64 bit isn't completely compatible with 8 bit. So if your favourite program has been through many reworks/updates there is a chance that it has a buried module with 8 bit code, so the software rolls over and dies. This is what has been causing all of the incompatibility issues between Vista and older software (unless you used the 32 bit version).
I stand by for corretion.