If it is bigger than a 20 gallon (not 20 long) then I would strongly discourage you to move the tank with anything in it.
Your best bet is to run down to Lowes and get a few buckets (they have nifty 5 gal buckets with skooshy handles grips -- fish stuff is HEAVY!). Fill one half way and put the fish in it (you'd be amazed how much the water will slosh!), use one or two for the gravel and stuff the ornaments, etc in those buckets also if they fit, being sure that everything is covered by water. It is critical that the filter be kept wet and, when you're refilling the tank, that the first several gallons be dechlored before they hit the gravel. (use the buckets the gravel travelled to your house in for this step - you can always pop in the rest of the neccessary dechlor and fill the remainder of the tank with a "drinking water safe" hose) Once the water is the proper temp, pop the fish in and give them a few days to settle in before feeding them. If the person you're getting your fish from could fast them for a few days before the move it would be great. I've never brought water with me when I've purchased functioning tanks and have never had a problem, YMMV. The only water I've had has been the water the fish travelled in and maybe a second bucket with decor.
Since you're only moving the tank a few miles, the tap water should be very similar unless you're all on well water (which should match pretty closely as well) or different towns which have dramatically different water sources. The reason I suggest buying new buckets is because 1. you're going to need some committed "fish buckets" for maintenence anyways 2. you need buckets which haven't been used for anything which might affect the fish. 3 miles is nothing for a move -- take your time and do it right, get your tank set up just the way you want now (and don't forget to leave space for the filter when you set it against a wall!)
Congrats and good luck!