Depends on the game you play. To take it to another extreme (just because you seem game for it) if you had an AMD GPU and mantle (and played a mantle supported title) the i7 is almost irrelevant!
I'd love to see some gaming benchmarks where any AMD CPU beats a 4790k in minimum or maximum fps. Go ahead and link some, would be nice to see.
Gaming is only one aspect. the OP mentions other tasks apart from gaming, which would also benefit from the I7.
There are only a handful of released games that support mantle, out of many thousands of recent games. DX12 will obviously add a lot more games in the future that will require less of a CPU for gaming, though it's not here yet.
Also - it's really hard to judge/advise someone on the best CPU, since we don't know their real usage of their PC. I imagine by now most people have multiple monitors, have multiple programs open at the same time that they are playing a game. Browser tabs, videos playing in some media player, some VOIP like skype, anti-virus, monitoring software, the list is endless.
The more programs one runs at the same time, the more the I7 will show it's obvious performance advantage - even if the game is mantle supported.
I imagine there are a few gamers out there who close every single process, program that's not essential to windows while they play games, to get the highest min/max fps, though I think we can agree they are in the vast minority.
Whatever way you look at it he's not really going to save in this scenario - which highlights the irony of your advice.
Obviously he won't save any money if he upgrades any component. We can bicker about the most cost effective upgrade for him, though we can't deny if he wants the best performance/lowest TDP/lowest electricity consumption all wrapped up into one package, he should choose the I7, assuming he wants to spend that kind of money.
Upgrade by spending £250 more on just a CPU - then the rest of the components from the switch assuming you can recoup reasonably by selling it. Oh and by the way you are going to save £2.31 this year by switching to an overclocked i7!!
Yes, he'd have to spent a few hundred pounds if he wanted to upgrade to the best mainstream (non enthusiast) CPU/Motherboard.
Regarding the money he'd save on electricity if he decided to upgrade to an I7, it would completely depend on what voltages he has to pass through his system to achieve a certain frequency, and depend on what frequency he wants to aim for. All chips are different, so it's very difficult to arrive at a certain figure.
You'd have to factor in how many hours he runs his PC, the average CPU utilization for each day, the region of the country he lives in (different prices per KwH), just to name a few.
Not idea how you factored all that in to arrive at your £2.31, I guess your simply clairvoyant, so I bow before you
Whilst it's true you won't that much money each year you run the I7, it's just one aspect that the I7 beats the FX in. When you understand that the I7 beats the FX in every single aspect, it's just one more slice of the overall pie. Of course the I7 is much more expensive than the FX CPU - though that's only natural. We're comparing Ancient FX technology (especially the features that the FX motherboards lack) to the latest technologies available on Z97.
Also bear in mind that this will all change soon - since Broadwell is just around the corner, and will further extend the hilarious lead that Intel has in every single aspect over the FX range (expect price of course
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As these others agree (just not you Dave):