1) If you dont need the mediadirect partition, you can just delete it. However, as you yourself said,you open yourself up to accidentally pressing the media direct button and causing problems.
What some people do to avoid that kind of accident is to remap the mediadirect button. See these links. Reassigning the mediadirect button- an adventure... [GUIDE] How to change your Dell MediaDirect button's functionality
2) Yes, you can boot from the Windows 7 dvd and do a clean install. If you wish, you can delete the mediadirect partition and remap that button. Leave the other partitions as they are, if you are constrained for space at a later time, you can consider deleting them.
Before installing Windows 7, make recovery dvds of your current install. That'll let you go back to vista, if need be.
3) Windows 7 will install many drivers on its own. You can then update them, in case there are missing or problematic drivers you can use the CD to install them.