Don't let that put you off Cosair RAM though, they are meant to be very good
It is possible for RAM to go bad though, and it can cause rather major problems to the stability of the system in general.
No, I don't think Brand in particular, though of course they dont say to buy different brands and it might affect the dual channel...and also with different brands and models, the latencys would be different which would again change memory speed between the two and might cause instabilites, so yes I'd assume so really. Though you always make sure you get the same speed!
Alot of Dual Channel RAM comes in packets of 2, and its best to get 1gb now to be honest with you, now times moved from 512mb as the normal amount