Use the highest purity Isopropyl alcohol (rubbing alcohol) you can, preferably 90%, but 70% is fine. It will remove the previous paste to give a clean contact surface for the new paste to interact with.
I replaced my thermal paste on both the r9 290 and 4770k. It greatly improved my thermals, but since my case fan speeds are based on cpu temp rather than gpu temp, the stock fan on the gpu was much louder without as much aid from the case fans. A little tweaking to my fan settings in AI Suite III fixed my issue.
On windows, I think 8 is good option if you have a touch screen. however, I have always had a lot more trouble using windows 8 since it is non-traditional with its interface. It feels like you are using a giant tablet. windows 8 has been shown to yeild more fps than windows 7, but it is insignificant (maybe 1 to 5 fps tops). I and many other users use windows 7 because we prefer the format and function of the OS over 8.