I have decided to bite the bullet and buy an ATI 5770 instead of the 4850. I have not bought a video card for a very long time so I was wondering about the different manufacturers. I'm drawn to the XFX because of the limited lifetime warranty and they have been around for a long time - good quality cards as I recall. I don't know much about PowerColor or HIS. I did a comparison between the different cards, but nothing jumps out at me, except that the cheaper PowerColor does not appear to have a display port, only one DVI port and has no user reviews as yet. At the moment I'm leaning toward the XFX HD-577X-ZNFC - I can't see what the difference is between that one and the more expensive one - except it looks meaner .
As you said they are all pretty much the same the main differences are the Fans\coolers used by various companies and the warranties offered on them - both HIS and XFX are fairly reliable companies - the drawback on the HIS comes if you plan to use multiple monitors or crossfire - it has only 1 DVI like the Powercolor card and has only a single connection for crossfire bridges so can be a problem if trying to use triple or quad Xfire configs since you can only connect one bridge.
Between the 2 XFX cards the enclosed one will vent more of the airflow out the back of your system instead of into the case so is useful for small cases with poor cooling - though the cards run pretty cool so if you have a decent airflow in your case the ZNFC model will work fine !!
I recently purchased the HIS 5770 because it had free shipping at the time. The only things is, make sure to get the cards with the egg cooler, it cools more than the shroud cooler, making overclocking easier. I think you really can't get a better 5770 than the ZNFC, unless you want to pay more.
From what I've seen they both perform about the same - the shroudded versions do tend to run a few degrees higher since the shroud limits a bit of the airflow around the card so they run a bit hotter compared to the rest of the system (since the shroud also keeps the rest of the system cooler by keeping most of the airflow inside the card or out the back of the system - where the egg shapes seem to run a bit cooler on the card but raise the internal case temps a few degrees due to the air that goes into the case instead of out the back ---- but overall I think they are about equal in a good system with adequate airflow.
Just one more question - I see people have been having issues with these cards and backwards compatibility to PCIe X16 - I have an older MB. Has this been fixed with the newer drivers or is this still a problem? I have a GA-965P-DS3 MB.