I bought the MSI Twin Frozr II OC on sale for $259 USD and am very happy with it (too bad the sale is over already, i might have picked up a 2nd one). It's SUPER quiet, in fact, I never hear the fans over my case fans and it runs very well. When gaming, even for a couple hours at a time, I've never seen it hit a temp above 62C.
I don't overclock much, but it seems to have decent headroom when i try. It's just barely factory OC at 810Mhz (vs.800 Mhz reference), but I've cranked it up to 900 without tweaking voltage and had no issues at all. I've had XFX cards in the past and like them as well (the double lifetime warranty is definitely nice). In regards to this specific 6950, I'm very happy with it, would happily recommend the MSI and will likely be buying a 2nd (maybe 3rd) in the next few months. Not sure about the size of the Twin Frozr III, but the Twin Frozer II isn't massive and fits into a wide selection of cases.
I'd also say that if your buying the Frozr III model because of the overclocks, it's not necessarily worth the money. The Frozr II has an outstanding cooler and can easily be overclocked for $20-$50 less (depending on where you buy and if it's on sale).
Hope this helps!