I had someone email me and say that if I'm going to get Windows 7 I shouldn't get 64 bit because there's almost nothing that runs on 64 bit.
Well you can email them back and tell them they don't know what they're talking about.
So long as all your hardware has x64 drivers most things should run on it fine, Win7 64 bit has great compatibility with 32 bit applications, while more and more software is being produced with 64 bit versions. I don't think it runs 16 bit applications, but it's very unusual to come across those anymore.
However, you should only upgrade to 64 bit if a) you really need it or b) you really want it, if you don't care either way then just install the 32 bit version. I'm using 64 because I have 4GB of RAM (4.5GB if you include VRAM) and I want to be able to use it all. Also, some performance tests claim that 64 bit is slower if you are using less than 3GB of RAM, so if you only have 2GB I would definitely recommend the 32 bit version.