Treachery,
There are a lot of calculators out there to figure out the answers to all your questions. Here is one of them. You can input whatever rim/tire/rear gear combo and most of the trans you are likely to use and see the rpms, mph, splits, etc. Thank Mike Stenhouse of SecondStrike for putting it up for us.
http://www.secondstrike.com/Technical/GearCalc.asp