The admission gates are around 3km from Siem Reap, and the temples are spread over a substantial area beyond the gates. Angkor can be explored by car, minibus, bicycle, motor-bike or even on foot -- however you choose to explore them, take your time -- see a couple in the morning and a couple in the afternoon and rest in between. Most opt for touring either by moto or remorque moto. Rates are up for negotiation, for motos in the past we've always got away for $6 a day, though the last trip paid more due to the continuing increase in the cost of petrol. Your driver will expect extra to take you to Banteay Srei or Phnom Kulen, though you should be able to wangle the Roulos group for the same fee.
Where possible, try your driver out before booking him for a few days -- gauge his language abilities and more importantly his driving abilities. If you are planning very unusual itineraries that will involve a lot of driving, mention it to the driver before-hand.
At the moment foreign tourists are not permitted to hire a motorbike in Siem Reap without a driver. If you want to do it solo, you need to do it by bicycle.