I have HD service with WOW; unfortunately I can't really help with your questions since I've always had digital service and the pricing has always been part of a package including internet service. It sounds like you already have basic analog service with WOW, have you tried hooking the cable up directly to the set to see if you can get the broadcast HD channels? I don't know if WOW encrypts all its HD channels or only the ones in its separate HD tier and the premium HD channels. If I understand it properly, if your built in digital tuner can receive QAM you shouldn't need a box unless the signal is encrypted (I don't think WOW supports Cablecards yet).
As far as I know WOW's HD boxes are digital, the SA 3250HD and the SA 8000HD DVR (which is supposed to be $12.99 a month btw).
On the plus side, according to info in their channel guide, WOW will be adding WB-HD and UPN-HD to their lineup in December.