To answer your questions in order:
***Can I have both IE7 and FF3 (or 2) installed simultaneously on my computer
Yes
***if so, are there any priority conflicts between them
Yes, each will want to check to see if they are the default browser. Just leave IE set to the default and tell FF no, do not set to default and do not ask again. This way, Windows is happy because IE is default yet FF can be used when you want to.
***when downloading FF3 (or 2), will my IE7 information automatically transfer over into FF3 (otherwise this could be very time comsuming to manually input everything)
Not when downloading, but during installation FF will ask you if you want it to import settings from IE.
***would I have to initiate a FF3 (or 2) email account or have my IE7 email re-directed to FF3 (or 2)
FF does not have email. If you're using your browser to access WEB MAIL through Gmail, Hotmail, Yahoo, or any of the other web based email accounts, FF can access it just fine. However, if you're using Outlook Express or even Windows Mail (on Vista), you will have to start those programs manually. There is no link between FF and any email program.
***is FF3 actually faster than IE7, and if so, by what percentage overall
That depends on the web site being visited. You'll just have to test for yourself and see. Based on what I've seen personally, using this site as a comparison, FF is not faster the first time the site is visited (in fact, it's slower), but subsequent visits SEEM to be faster.