A few quick notes:
First check if your PC and its components are on the Win 7 (or 8) HCL, and then check the same for peripherals, then apps.
You can also use the Win 7 Windows Experience Index to see where your current system is in need of upgrade. Note that in the graphics department, you can simply turn off the whole Aero interface.
There are a couple of User Migration tools that you can run to migrate settings, but don't expect them to be comprehensive - you will have to run them as each user, and even then it's probably wise to keep all the files in "Documents and Settings" available after the migration.
What I did was to get an extra hard disk, remove the XP disk and install Win 7 on the new disk. That way I found most of the problems and gotchas. It works best if you already have separate disks for System / OS and data.
List all the cards, components and peripherals (as much detail as you can get - make, model, revision) in your system before you start, so that you know what drivers you are going to need to get hold of - much easier to use XP Device Manager than open up the PC to check!
SD