1) Know the specs of your computer, like RAM, CPU, etc. Failing that, know how old your computer is. If it's going on ten years old or something, it's probably not a good idea to go to Windows 7.
2) Know if the programs you're running will actually work with Windows 7. Many do, many don't.
3) Know if you want to install 32-bit or 64-bit Windows 7. The main advantage of 64-bit is being able to use more than 4 GB of RAM in Windows without workarounds/hacks/etc. The disadvantage is that ALL of your hardware must have 64-bit drivers (scanners, printers, and whatnot).