When you boot the Win7 DVD it will see XP on the hard drive and allow use of Upgrade version key, even if you then use Custom>Drive Options to delete XP, create New partition(s) and Format before install.
If you pre-format the HD it requires using one of the workarounds shown in tutorial to use Upgrade version key.
The installer is mostly driver-complete, with newer drivers arriving quickly via optional Windows Updates. Any drivers then missing in Device Manager can be found on the Support Downloads webpage for your computer model, or the device model.
Do not change drivers given by the installer or Windows Update unless there is a performance reason.