Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.
I've went from a i7-2600k overclocked to 4.6 ghz to a 3770k @ 4.4 ghz and now a 4770k at @ 4.2 ghz because of heat with same H100i cooler. Honestly the 2600k is a beastly processor that can OC very well and keep up with the 3770k no doubt. 4770k kind of pulls away but its no sacrifice to gaming...
Paying full price of course its not convenient thats why I mentioned if you can get a good deal on it. I recently got a 380x for alot less than $220 and I am sure you can find more like that if you look around as well as a GTX 970 in a really good price range way under the price of a new R9 390.
Best bang for the back which those requierements would definetely fall on the GTX 750 ti FTW card which you can get for a pretty good price used or a GTX 950.
The R9 380x is actually a little bit faster than a GTX 770 which is a pretty good performance gap from a GTX 760 if you get a good deal on it. If he can go for a GTX 970 even though its a little pricier I would recommend that more, you can pick one up for a really good price in mint conditions...
I would go with the GTX 760, the extra 2GB VRAM in that range of card ain't really a big difference but its nice to have. The 680 is definetely too old and those reference cards do get pretty hot so I doubt it has much more life in it and the GTX 750 ti just doesn't have the power. That 760...
There are some great deals on GTX 970's here on the forum, has great performance and low power consumption which is perfect for your setup in my opinion, haven't needed more than a 970 since I only run a 60hz monitor.
With your budget you can definetely find a decent GTX 980 around there, I got a reference design for $340USD shipped which is a little under $500CAN a couple weeks ago and it does marvels with Dying light.