A: At the moment, versions 2.4 and 2.5 work the best for R5xxx and R6xxx GPUS. SDK 2.6 or 2.7 works best for R7xxx. SDK 2.8 is known to have many problems. If you are need to use the 2.6+ SDK or R7xxx or later, the phatk kernel will perform poorly, while the diablo or my custom modified poclbm kernel are optimised for it.
Derp.
I tried installing 2.5 since I have a 6870, but cgminer (whether using the prebuilt binary or one I built from source) segfaulted a second or two after startup. I think it was a libstdc++ conflict of some sort. As things stand now, I have v12.10 drivers, v2.6 APP SDK, and v5.0 ADL SDK installed, and cgminer 3.1.0 built from source. I let it mine against EclipseMC for a few minutes...saw that it was getting a few shares accepted. Since other coins are currently more profitable to mine, I'm not currently mining Bitcoin...but if the bottom falls out of the different altcoins or if an sha256 altcoin becomes more profitable than an scrypt altcoin, I can switch over to that now.
One thing I noticed: the fan runs at a much slower speed when doing sha256 mining. While keeping the GPU at about the same temperature, the fan was running at about 1600 rpm or so. Doing scrypt mining, it usually runs at about 3400 rpm. Temperature is about the same either way: 65-67°C in a cheap midtower case with the lid off (lid on raises the temperature to around 80°C). The fan on the 7750 in my main mining rig behaves similarly: fan runs (IIRC) at about 40% or so when doing sha256 mining, but at 85% when doing scrypt mining.