my 2cents would be to stick with a quadro k4200 or above. its a decent upgrade over the k4000. your gpu may be the weakest link for the hardware currently. if you need to save money and dont need the quadro then a gtx 960 or 970. if you need the quadro for a reference monitor, you may be able to run your current quadro as a dedicated display output and a new gtx as the cuda/render card...
i cant think of any reason to upgrade to win8.1. also i believe microsoft announced that they are giving away windows 10 for free, to windows 7 and 8 owners, for the first year. if thats true, you can go that route.
memory and hdd's really depends on your media and what your doing in adobe cc. faster hdd's will almost always be welcome, while depending on what your doing, and your footage, the ram may or may not be worth the investment. if you do get the two wd blacks, you might be able to run those in stripe/raid0 and then use the greens as backup. possibly by creating another stripe/raid0 with the wd greens and using a file synchronization program to copy any new/changed files to the wd blacks to the wd greens at the end of the day.
