The PCI-E slots in something like an 1U R630 or R640 are x8 or x16, the problem is more with the cooling and size/shape of the cards and how they cool themselves. The slots are meant to be used with things like 10 or 100 Gbps network cards or SAS/SATA host adapters which are considerably lower wattage than even a video card that's much weaker than an intel 770.
Commonly you will also find configuration with two or three 'low profile' pci-express slots which have a different card height than the 'standard' height that most GPUs are built at.
that works for crypto because all the CPU sends to the card is the target ledger sha256sum (simplified) and the GPU generates nonces until the `sha256sum(sha256sum(nonce += ledger sum)` has however many zeros in the front. So until a card finds the correct nonce, or the server sends "new work" - a new ledger shasum, there's no traffic, really, between the GPU and the CPU. housekeeping, whatever, but not like 1GB/s!
Commonly you will also find configuration with two or three 'low profile' pci-express slots which have a different card height than the 'standard' height that most GPUs are built at.