If you plan to overclock your i5 grab a cooler master hyper 212. Its cheap and does a great job.
You should be good with a 650w PSU for a 7850. Says 550w is the minimum so it give you some breathing room for everything else. Just make sure you dont get a real cheap PSU its wise to invest in a good one. Corsair, Seasonic, Antec all make good PSU's.
Case wise just grab something you like thats cheap and has good airflow. The Antec 300 is a popular choice. Pick your PSU out first then figure how much you got left for a case.