Now that I'm actually running the 64bit version of Windows 7, I can actually answer some of the queries and concerns that people usually have about a 64bit OS.

If I have 4GB RAM or more, I need 64bit?
This is pretty much a straight on yes, a 32bit OS will never be able to fully utilize 4GB or more of RAM. While you can enable PAE or some other fixes on a 32bit OS to work with 4GB of RAM. A 64bit OS is the best  chance of using your RAM to it's full capacity if you have >= 4GB of RAM.

All my old 32bit programs won't work on 64bit?
The answer to this question is a MAYBE. Generally if you ran a 32bit program under 64bit Windows, it should just work thanks to how Windows is designed. BUT... if the 32bit program does something funky with memory or assumes that it's running on a 32bit OS. Well, that's when bad things happen. General rule of thumb is that normal 32bit applications like office, most games, browsers will run normally under 64bit Windows. Anything that's system related like Antivirus, system utilities or certain online game copy protection root kits, will need specific 64bit versions.

All my old hardware won't work on 64bit?
This is a YES, only because in order for any hardware to work on 64bit Windows, you NEED 64bit DRIVERS. Hardware in recent years *should* have 64bit drivers already. But if you had something very very old, like say an old 10 year old Canon scanner. Well... that'd be a problem then. This is generally the biggest hurdle for people to go 64bit.

It's actually quite possible to go 64bit now because there's better driver and program support to make sure the things don't go crazy when running under 64bit. But if you have old hardware, you'll want to check to see if they do support 64bit first.