I was considering the 32 gb card, but there isn't really much point as I'll either download or delete the footage each time I go home, and the battery only lasts 2.5 hours. I would say an 8 gb card is probably sufficient, unless you get the 32 at a great price.
Recently just got a GoPro HD, might get this aswell and compare the footage and just use one in the car for "evidence".
What sucks is that it uses MicroSD... i bought a 32GB SD just for the GoPro too...