The performance is similar, no its not, 5870 is hugely faster than a 580gtx, like, 50% average maybe, not even the same ballpark.
Noise though I'll give you, the fans are silent but any increase in temps, two cards too close or warmer case and they start to get irritatingly loud.
Have you got yours in slots next to each other so one card is almost blocked by the other, any chance you could put the 2nd card in another slot 8x pci-e slot wouldn't matter, not all boards have the option. 3rd party coolers and xfire can be an issue unless you have a mobo with great spacing of the 8/16x slots.
As for a 580gtx, stock fan is supposed to be pretty quiet, not as quiet under load, the hotter your case/room/overclock the louder that fan will get.
THe phantom, gainward version is supposed to be quiet, and I'd expect the ASus CU version to be quieter(but for such a big sink and two fans my 5850 version wasn't particularly quiet under load), MSI twin fan jobby will likely be your quietest versions though not sure on cost.
If you want something quiet then
http://www.overclockers.co.uk/showproduct.php?prodid=GX-143-GW&groupid=701&catid=1914&subcat=1010 that seems like the best bet.
A 570gtx isn't much slower than a 580gtx, both get spanked by a 5870 xfire, both would have similar raised noise issues if you went with SLI with a couple of either.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-115-MS&groupid=701&catid=56&subcat=1866
Something like that in xfire is likely the best bet performance wise, hmm, thought it was a 6870, you lose gpgpu functions but get quieter, still very fast performance, 2x6870's definately out perform a 580gtx, I believe 2x6850's do aswell, decent quiet cooler.
OCUK don't have a great option of 3rd party cooled 6870's so you might look elsewhere for something similar.
Obviously switching to 2x5870 with third party cooling IF you can find some cheap 5870's with 3rd party cooling, or looking at something like a Zalman cooler for each card, but that would only work if you have spacing for the cards.