Although I am a little strapped for cash right now I really want to upgrade my system to be able to multibox Diablo 3 and to keep up to date in the event I find another game worth playing. Preferably aiming for mid-high graphics in games like Skyrim, FarCry 3, and if I can, in emulations as I love to play ps2 games on pc.
Just let me know what are some of the easiest ways to improve performance like gpu or mobo/cpu combo or even an SSD.
This is a tough one. You could benefit from a CPU and GPU upgrade, but GPU usually always trumps a CPU upgrade. I would honestly start with a GPU upgrade. Your 600w Thermaltake should handle a pretty decent upgrade, maybe an Nvidia GTX660 or AMD 7850/7870. It all depends on what your budget is. The cards listed above are where I would really start. Anything less would be worth just saving until you could get a good upgrade. That will give you the best bang for your buck. Secondly, your system really has a good upgrade path as far as CPU. You could drop in a $140 AMD FX-6300 and be done with it. Everything would just work. Since you already have a quad core CPU, I think this would be a second step and even the Athlon x4 wouldn't hold your system back that much with just a GPU upgrade. However, games that are CPU heavy would really benefit from the CPU upgrade. The beauty of the AM+ board is it has a good upgrade path.
Diablo 3 and FarCry have very different requirements as Diablo has lower requirements than FC3. An FX-6300 with GTX660 would be a worthy system. Also, I saw on Newegg the gtx660 comes with $150 in came coupons for 3 Free to Play games, all of which are really cool games. The 7850 and 7870 come with a free coupon for Bioshock & Tomb Raider, which is a REALLY cool deal. Both those titles look really sweet. Either way you are getting either $150 or $120 in free game stuff.