I just installed my Sapphire 5970 from Newegg a couple days ago and here is my setup:
Gigabyte X58A-UD5
Core i7-920 @ 4.0ghz
Sapphire Radeon 5970
Antec TPQ-850
In my case, I have 7 hard drives so I didn't really think going CF or SLI was a wise choice for me. I wanted the most powerful single card solution possible, and the 5970 was it.
To give you some perspective, if you have some nice airflow, it idles at 40C and has a TON of overclocking headroom (with a little added voltage). Despite its mild clock speeds, you can easily match 5870's (850/1200) numbers. Even OC'd, with good airflow it peaks in most games in the 70's and low 80's, which is safe for this card.
It wasn't the doomsday be all end all solution I expected, like for example it still struggles in 1920x1200 Crysis with everything Enthusiast and 8x AA (max everything) producing just around 30fps. I actually switched everything back to Gamer to a more playable 40-50fps. It was kind of a kick to the ego just having spent so much on a card and not being able to max everything out. But hey, it's a lot better than the Radeon 4850 I replaced.
Fan noise is quite bearable imo, and I personally think I made a good choice. Sure I spent $699, but it's better than spending $1000, having to get a new PSU, and possibly a new case (I stuck mine into an Antec 900 - yes it's possible!).
Bottom line, just like the other nice folks have been saying, if you're gaming at at beyond 1920x1200 and you want everything maxed, money is no issue, you can power and cool it...then go with the Fermi's....otherwise, the 5970 is a solid bet.