Adele,
Less radiation is always better as radiation absorption is cumulative over a lifespan. The amount of radiation you recieve from dental x-rays is low in comparison to other types of x-rays. Check out the sites Gary has posted, I did, and they are accurate.
Digital dental x-ray sytems do allow for lower dosages of radiation to achieve the same diagnostic detail on the films produced. The previous statement is only true if the x-ray machines are properly callibrated for the digital system being used.
I use digital x-ray technology in my office for all intra and extra oral x-rays. I have had my machines calibrated by the company that I purchased the technology from and have them check my machines regularly. I know that I am using much less radiation per film than I was previously. Testing is only voluntary in my region.
I am aware of offices in my region who utilize digital x-ray technology and have not been tested. If a technician didn't callibrate the source at the x-ray machine these offices may or may not be using less radiation even with digital x-rays.
My best advice if you are concerned about radiation from dental x-rays would be to talk to your dentist. Not a week goes by where I am not questioned about the necessity of x-rays. I am more than happy to provide information and answer any questions.
If you like your dentist and value his/her expertise and skill; changing because they don't have digital x-rays, in my opinion, will not be of benefit. The x-rays denists take, whether digital or not, provide important iformation that is essential in diagnosing and treatment planning. If after discussion and providing up to date information on why I belive x-rays are necessary a patient still refuses I do not treat them. (I have one and only one exception to the above; if the patient is or has had radiation treatment and is getting close to or has reached their recommended lifetime dosage I will agree, after consulting their RO, to look after them).
Anyway...I'm way off your the topic of your question...sorry.

The change in dosage of radiation between digital and conventional dental x-rays, in my opinion, is not a good reason to change dentists.

Cheers,

Mike