When you say Go-Pro to use as a "dash cam" do you mean you want a sports camera for filming specific events like track-days etc (Drift HD, Go-Pro, Contour HD etc - all £100-400) or an accident cam like the Russian dash cams which record constantly on a loop and automatically saves a 1-2 minute clip when there's been an accident (£50-100)?
There's a thread in the Motors section about the Russian dash cams which is very informative if that was what you wanted.
If it's a sports camera then I've used all 3 of those I listed and I found that the Drift HD series suited my needs best (rotatable lens etc) although all 3 were very good. However none are suited to being used as an accident dashcam due to battery/memory usage filling up within 3hrs at most!