My second 'real case' mining box (now building an open frame rig - makes much more sense) had a logic board for 'crossfire' with four PCIe slots. With two 5850s in the twin x16 slots, one of the two remaining x1 PCIe slots was wasted and the other one could just be seen between the cards. Putting anything in there would kill the 5850s from airflow... so with two x1 extender cards daisy-chained together (it works), I decided to put a small old 5670 card in the vast empty space of the drive bay, just for a laugh.
I know these cards are slow, but my calculations suggest that at around a core clock of 900 MHz, all the Radeons I've used generate Mhash/sec as a quarter of their stream processors. So a 5770 with 800 SPs will give around 200 Mh/s at 900 MHz core. A 5870 with 1600 will give 400 Mh/s.
Hence a lowly 5670, with only 400 SPs, should get 100 Mh/s, right? It's not much, but the card requires no more than 75W because it has no additional power supply connector on the card.
To be safe, even though I'm sure it didn't need it, I used one of the x1 -> x16 PCIe extenders with the Molex connection so the card wasn't pulling all of its power from the logic board. After all, the board had two overclocked 5850s in the other two accessible slots.
With the card in the drive bay, held in with zipties and a self-tapper into a fan corner (!), I put another small blower fan in the front of the tower case so air blew across the card and out the back along with the rest of the hot air.
Using my usual Ubuntu 11.04 natty miner setup, the third GPU was effortlessly detected, and is now completely stable at 900 MHz... and is producing 103 Mh/s. Not impressive, but it all adds up doesn't it!!!! The card runs cool - this is at a steady 46.5˚C.
I'd best update the Mining Hardware Comparison wiki because that's more than anyone else has got from the card...