If your operating system is on one drive and all your data/ software/ medis etc on another, then you'll be formatting only your OS drive, which will not affetc your media. But applications like games, video convertor's, etc will need to be re-installed. All your movies, pics, songs, documents, etc will remain unaffected.
For eg, I've done multiple re-installs from 32-bit to 64-bit for this very purpose. However, what I've always done is partition my drives. So if i'm using for eg, a 500 gb drive, iI partition the drive into 2 parts. 200 gb dedicated to the OS only (C:\) and the remaining for other stuff (D:\). Everytime I've re-installed a different version of windows, all my applications which run into the registry get affected. These include games, anti-virus softwares, vidoe convertors, etc. However, my 'data' (movies, music, pictures, documents, etc) which do not run into the OS registry remain unaffected.
From my frequent switching, I've come to prefer 64-bit (with 8 gb DDR3 RAM) for the simple reason that it speeds up you operating system. While it doesn't give any improvement in games as far as quality or FPS goes, the load time has reduced incredibly! Also, my OS based programmes have started running much faster (video conversion for eg, which used to take like 10 mins per video, now only 4-5).
It's the additional RAM that a 64-bit OS utilizes which makes the difference as opposed to a 32-bit, which soaks in only upto 3.5 gigs. So yeah, I'd say you go for it. The re-install will be well worth the effort!