bellend,
In order to fly in the US you will need an FAA licence issued on the back of your JAA PPL. The FAA website explains how you go about the process but basically you need to pay a fee to our CAA in order for them to relase details about you to the FAA.
You will also need to tell the FAA which FSDO (Flight Standards District Office) you wish to visit (there is one in Orlando) in order to collect your Temp US licence. You will need to take your licence (JAA), logbook and passport with you when you attend. Please note - the FAA website states that you should allow 90 days for processing your application. If your going in September you need to get your skates on!!
Visa - you don't need one. You are not training therefore you can fly under the visa waiver programme.
You will need to do a BFR (Biennial Flight Review) with whovever is renting you the aircraft. The BFR consists of 1hr groundschool and 1hr flying and is similar to a mini skills test in the UK. Use this flight (and book additional time if need be) to get up to speed with the RT, local area and circuit procedures.
Rental costs will depend on where you hire from, I paid $100 per hr (wet) for a PA28 in March this year and a similar amount for a C172. Avgas was approx £2 a gallon but the price may have increased since then.
PD