That depends more on the device itself. For any addons like your video card, sound, tuner cards, printers, etc. especially more recent models you then have to locate what isn't found in the Windows updates as far as the application updates as well as device
drivers.
The Windows updates will typically see the
driver sets but not the application side like the AMD/ATI Catalyst or NVidia Desktop manager. The sound mixer app for a separate sound card would again be found at the manufacturer.
As far as the larger amount of generic
drivers found in 7 over XP take a look at how many years have passed inbetween those two versions as well as how many newer hardwares and devices are now found on the market. External self contained hard drives, usb devices like flash drives, cell phone SD cards acting like flash drives the same way the onboard or added in memory card on digital cameras work. The development of 7 included support for these newer technologies not in existance back in January 2001 when XP was first seen.
One other item that will be a great help however with any clean install of Windows is going for the Secunia Personal Software Inspector which will prompt when any newer version or update for anything installed is found. Even after Windows has been on for time this has proven itself to be an asset.
Free Computer Security - Personal Software Inspector (PSI) - Secunia
Besides being a security tool that will also make sure the
drivers that go on are the correct ones for what you may need to find once you have to go with a clean install of Windows.