Compared to the Sapphire dual-slot 5770 with a similar heatpipe cooler to the 5850 'extreme' - it runs 5 degrees hotter and overclocks 100 MHz less - so inside a case, I can get 230 Mh/s from the dual-slot Sapphire and 200 MH/s from the single-slot XFX brand card.
The good thing about this card is that if you've got a random computer lying around with the typical three PCI Express slots, all in a row, with the x16 at the top for the GPU, then two x1 slots following each other, then you can build a safe, low-temperature dual-GPU system in the case without resorting to extender cables or butchering the case. A bog-standard computer can be used as a dual-GPU miner...
All you do is put the single-slot 5770 in the top x16 slot. This allows both x1 slots to accept cards, but of course you won't want to put another GPU in the slot *immediately adjacent* to another GPU, as it kills airflow and will overheat the top card. Instead, skip a slot and put another GPU in the bottom x1 slot (you need to Dremel the end off so the card will actually fit, and the logic board better not have any big components blocking the way, but slots at the bottom of the logic board are usually pretty open).
The usual method is to put the powerful GPU in the x16 slot... but this both covers over the first x1 slot, making it unusable, and leaving one open x1 slot too close to the main GPU. I've tried two cards immediately adjacent to each other, and it's unstable and hot even with monster fans.
Using this method, all you need is a Dremel and some patience, and a cheap old 3-PCIe logic board in a cheap case (there are loads of < £50 second hand computers with this sort of configuration) can be made into a auxiliary miner. It's not ideal as a main miner but if you've got one lying around...
And the good thing is that you don't need a monster PSU unless you put a monster card in the second slot. With the single-slot 5770 running at 200 Mh/s (mild overclock to 900 MHz), and a 5850 running at 380 MH/s (mild overclock to 925 MHz), my test *entire machine* pulls around 400W from the wall, using a power meter strip. This old PC also has three big fans (one pulling air in from the front, one exhausting at the back, one blowing in from the side onto the GPUs) which draw a few watts each from the 12V rail. So I'm using a random 500W PSU I had lying around - it's in the efficiency zone at 400W *total* draw, not in the danger zone, and makes good use of the crappy old computer without wastage.
Cost-wise, it's a crappy old computer (you may have one lying around), say £50. Single slot 5770, £75 from overclockers. Dual-slot 5850 'extreme', £120 from overclockers. Three quality fans, £30. Reasonble 500W PSU (you may be able to use the one in the PC), £40. Assume you've already got the Dremel and some tools... total cost £315 for a cased-up, tidy 580 Mh/s eating around 400W at full load.
Probably not the best, but good use of equipment otherwise unusable for more than one card without extenders. A tidy enclosed case can be placed safely anywhere even where inquisitive cats exist
And such cases are also more acceptable to one's partner than a bare-board hacker miner if placed in the study / spare bedroom / kitchen / etc. 
