DXG 521 Roadster is a poor choice imo. I bought one also for recording from my motocycle, got a nice bullet cam setup too.
The recording quailty from the internal camera was fairly good, chrisp, sharp and had consistant bitrate, soon as I plugged in my bullet cam all this changed, quailty wasnt anywhere near as good, poorly pixalated and the bitrate jumped all over the place, recording at 640x480 max quality would sometimes fill up a 2GB memory card in 15mins or other times 45mins (Using the internal camera, which has better quaility, would last atleast 1hr30mins before the SD card was full)
Another VERY annoying bug was 90% of the time for no real reason the whole video would get blury and stay that way until you rebooted the machine. I found a way to reproduce the bug 100% of the time. Basically you point the camera at a dark place then quickly to a light place e.g. inside an unlit room, then quick point it outside on a bright place by the time it's come back again the image quality has halfed. I even plugged in my proper camcorder and it produced the exact same results.
I've emailed DXG many times, sent them video clips, screen shots, took details of bitrates. They actually turned around and said it was perfectly fine, The guy was a muppet, he said it was an focus issue and made up all other stupid excuses too. To me it's all software issues, they just didnt bother testing the TV-In functions but they wouldnt admit this or issue me an updated firmware.