IF you don't know anyone that can help or you can't do it yourself:
Chances are your probably better off with buying a next-gen console. I am a computer gamer, but with that current computer you would need to buy a decent dedicated gpu. Considering what your wanting to play and at half-way decent settings with no drop in fps and also assuming your playing at 1080p. After the cost of the upgraded parts and installation fees your probably looking at spending more than on a next-gen console. Primarily due to the installation fees.
IF you know someone who can upgrade the machine or you can do it yourself:
I would go with an Nvidia GTX 760 for a little future-proofing. Granted it's not the most powerful gpu it's a good mix. I am not a fanboy, but prefer Nvidia for their driver support and the fact that most of my games seem to like Nvidia better. *shrugs* That shouldn't cost you anymore than about $270-$300 TOPS after any shipping charges. Kudos on finding it cheaper if you do. (I currently own an AMD R9 270 Overclocked and it does decent, but I really wish I had bought a GTX 760 or GTX 770.
For a psu and working off a few assumptions, I would say a good 500W would be ok, but pushing it on if you wanted to upgrade down the road again. I would go with a decent 600W PSU. Make sure it has 2x 6/8 pin pci-e power connectors though. Seasonic, the higher end Corsair would do good.
Quoted from another forum, but can't find it this is a copy/paste from my notes:
"CX is the budget series (non-modular, 80+). TX is mid-range (non-modular, mostly 80+ bronze). HX is their professional line (modular, 80+ silver mostly). The AX is the top of the line (fully modular, 80+ gold). There is also a GS line which is gamer series which have LEDs and are not modular."
I have a CX600M that seems to be running my AMD build ok (AMD FX-6300 stock clocks, 16GB Ram, 2x HDD's, AMD R9 270 Factory-OC, about 30W's worth of fans. It cost me about $70 with prime shipping.
Here I found a suggestion:
PSU:
http://www.amazon.com/gp/product/B00ALYOPSS/ref=oh_details_o04_s01_i02?ie=UTF8&psc=1
GPU:
http://www.amazon.com/GTX760-DC2OC-2GD5-GeForce-256-bit-PCI-Express-OC-selected/dp/B00DBPKEOI/ref=sr_1_2?s=electronics&ie=UTF8&qid=1403443296&sr=1-2&keywords=gtx+760
You would also need to watch the size of the gpu as your case may be too small. I picked the smallest GTX 760 I could find. This was based all off of what I game at and what your wanting to do. I'm currently looking to 'upgrade' my gpu to an Nvidia equal or better than my R9, so yeah.
Dayz would be able to play on Ultra with that gpu and cpu combo. It would cost about $330 if you have Prime. May have to tack on shipping charges, but that is a lot less than a next-gen console.
Your cpu should do good for awhile and ram is a cheap upgrade down the road, but with 8GB already your pretty will set. Though if you do a lot of multi-tasking I would look into having 16GB of ram. (I constantly have about 10 tabs open in chrome and other things running and would get close to maxing my 8 GB of ram, so I upgraded to 16GB of ram.)
Predicted: you should be able to play the new Call of Duty: Advanced Warfare coming out in November on "High" or maybe even "Ultra" but that might push it. Still it's not a bad upgrade for something projected to come out in about 5 months.
Hope this helps.
Note: this is only my suggestion and not a promise that things will run smoothly in 2 years.