The different 560Ti have different cooling systems and come with different factory overclocks. If you want to do heavy overclocking the Hawk is the one to go for since it has the best cooling of the three. If you don't want to overclock here are the clock speeds they come with from the factory.
Twin Frozen 880MHz
I personally have a MSI GTX 460 Hawk and am very happy with it. I have it Overclocked to 33% above reference clock speed and never get above 60C while running at 100%. 33% above reference for a 560Ti is 1095MHz, for reference reasons. All of the cards you listed are solid, and have above average cooling, so you really can't go wrong, though I would say the Asus and the Hawk are the top two.
Thanks for the advice. any idea how does Ti 560 Ti stack up against 6950 2GB especially for graphics intensive games like skyrim? does the difference in performance justify a 30-50 USD difference in price?
What is the resolution you are playing at. 1GB is fine for most games at 1920x1080 or below. It is when your resolution is larger than that that you need more than 1 GB of ram.
If you are playing at 1920x1080 it becomes a toss up between the 560Ti's at 1GB, and the 6950's at 2GB. At larger go 6950's and lower 560Ti.
Wait, is that 6970 a typo? Even though it is a Sapphire card, that is mighty temping in comparison to the other cards. It is by far the most powerful card of those listed, and at the same price as 6950's. It seemed to have good reviews on Newegg. I've never looked into Sapphire cards so I am not sure what their quality reputation is, but still I am very tempted to suggest that one. Get a second or third opinion though before grabbing it.
Also make sure your power supply can handle the cards, and that your case has enough room. Some of those 6950's/6970's are HUGE.
My GTX 460 1GB doesn't have problems with Skyrim (on high/ultra) even with 3 hour sessions. I think if you modded it a ton with high resolution textures you might run over 1 GB of video ram.
If you are going to play at 1080p I might suggest going with the 6950 from MSI or Asus. That way as new games come out with higher quality textures that use more ram you will be covered, making Crossfiring the cards down the road a better option than SLI the Nvidia cards that are running out of RAM.
If your motherboard doesn't support Crossfire or SLI I would just grab the 560TI Hawk and overclock it a ton.
For power supplies, I would grab a good quality 600W power supply for one card, but if you are going for SLI/Crossfire bump it up to 750W or 800W. It can't hurt to oversize your power supply slightly.
OK I am now recommending going for a MSI or ASUS 6950. I've been messing around with texture mods for Skyrim. To use the good high quality 2048x2048 texture mods you want at least 1.7 GB of video ram according to the mod makers. I tried the high res texture mods with my 1 GB 460, probably overclocked enough that it is close to a stock 560ti, and I was getting a slide show in some locations. I had to downgrade to 1024x1024 resolution textures (which still stomp over the 512x512 default textures)
hi the sapphire 6950/6970 are few of my friends have them all on CF setting, both those cards perform well and they play at 1 monitor and do some autocad/3d renderingand music stuff on another monitor, and he plays bf3 with decent fps, although i have another friend who uses 560ti hawk still going good for now but not sure yet since he still haven't installed bf3/metro2033/skyrim 2 with his 560ti hawks, and don't know if its reputation to get extreme heat will heat up his system or his normal cooling with lvl 10 gt will be good enough