Yes, you can use PCI-E x1 for the x16 video cards since the OpenCL/Mining applications aren't host<-->gpu intensive.
The caveat is that an x16 wide slot can supply up to 75W of power and the x1 slot is not designed to do that, so it's recommended you get powered risers (pulls the slot power from the PSU directly instead of through the motherboard) to prevent trace burnouts, smoke, etc.
... so I can drop in 5 cards (two x1 slots and three x16 slots)? Oh this is about to become very fun.
Thanks for your response CryptoMinter
You
might be able to. Physically you can install 5 cards. That is a given. The issue is with the motherboard. Some motherboards shut off certain slots when other slots are occupied, or some motherboards run out of memory preventing all of the cards from working properly. You are probably just going to have to try it. I would recommend ordering the risers and borrowing some extra cards from a friend or something, even if they are cards that wouldn't be good for mining. If the system boots and the cards work, then you can order some proper mining cards.
