Between 27-32 weeks is the best time to have one done. =)
Hope that helps.
For Anyone interested, there is a new 2/3/4D baby Ultrasound Studio. It is located inside the Office Buildings at the Coconut Creek Promenade in Coconut Creek. It is not far from the Sawgrass Expressway or Turnpike. They just opened and they even sell heart beat teddy bears. You can have your babies heart beat recorded and placed inside a teddy bear. When you sqeeze the bears stomach you can hear your babies heart beat.
Packages start at $75 I think.
They are called UltraScan Imaging