Stay away from the cheap Chinese cams on Ebay. I had 2 before I reached that conclusion. The quality is piss poor, they will not (at least the two I had) will not cycle recordings. They are hopeless in bright light or low light They just stop writing to the SD card when it gets full. You would only know that if you went into the settings to check. I have a Mio Myvue unit now.
http://eu.mio.com/en_eu/mivue-388.htm
It has an approx 8 hour record onto its 32 gig sd card, then starts over recording old recordings. I spend 4 hours a day driving, so I get two days before it starts to overwrite. Clarity is incredible, even in night conditions. It auto detects emergency braking (it has a motion sensor) or impacts and saves that portion of recording so it does not get overwritten unless you tell it to.
When parked up - the camera is off..BUT, whilever there is a power feed to the camera (cigarette power lead).. It still saves about 1 minute of recording in live memory when parked, so anyone walks in front of the car, or if anyone bumps the car - you not only record the incident and aftermath, but the build up to it too.
About 80 quid from Halfords 18 months ago.
It already paid for itself when someone bumped my BMW while it was parked in a hotel car park. To be fair, they owned up to hotel recption - but how much of that was honesty, and how much was the fact they would see the camera in the front windscreen (and rear screen) I cannot be sure. I had all the detail recorded though ....