Questions -
- I'm tossing up between the GTX 760 2/4GB & GTX 770 2/4GB. Reviews say 4GB doesn't get used (will it?), & the GTX 770 is +$100 on my current setup, and another +$50 for the 4GB versions of each.
Assuming you're not playing any higher than 1080p, 4gb is fairly useless, 2 would be more than enough. I just bought a 770 2GB in my build and I'm really happy with it, handeled everything I've thrown at it.
- Opinions on the motherboard? I've taken a stab at a mid-range model here. While its newer than the z87, it does cost +$50 more.
No experience with the motherboard so I can't really help you there, Asus are generally a good brand though so I assume it'd be fine.
- Is 8GB of RAM fine? For gaming I don't believe 16GB will get used
8GB IS enough, more ram would make general use a bit quicker, but for the most part you'd get by fine with 8.
- Is the i5 processor fine for gaming? I think i7 is overkill (not doing video editing, photoshop). Also I've gone for the low power version, saves ~20W. I could get a i5 3.5Ghz for $10 more, but power saving is a good thing right?
yeah i7 isn't worth the price to perforamance upgrade from i5 (Unless you're just a nerd who wants to brag like me

). I would personally go for the 3.5GHZ, but saving power is nice too. Up to you.
- I've gone for a 750W PSU here. Put it into a calculator and it came out as a min. 650W with GTX760 & min. 750W with GTX770. If I get the 770, is it too close to the minimum & should I upgrade to 850W?
850W sounds a bit overkill, I have 700W running a gtx770 and i7 so I think you'd be fine.
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Your general thoughts & opinions are welcome

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That'll last you this console generation fairly comfortably!
(Sorry I'm not that much help, JC will probably be more help.