Receivers
I would recommend the Yamaha HTR-5960, read the recent AH review. You should be able to find it online from authorized dealers for just over $300. I have last years model, the 5860, which you can find for <$300.
Keep in mind that most receivers are not desinged to run the A AND B speakers at the same time. Doing so can cause the receiver to overheat and shutdown. However, the surround and surround back channels are avaliable for a couple extra pairs of speakers.