Future proof gaming build for $1200

There's no such thing as future proofing a PC, 3 years later and any $1200 build is slow compared to the latest and greatest.

Overclocking?? GPU preference, CPU preference, RAM needs?? Resolution?? Cooler preference?? Do you already have a case?? Does electricity cost a lot (a platinum PSU can save money in the long run) where you live?? HDD, motherboard preference?? how many PCIE slots do you need?? Do you need an OS?

I'm not sure what storage requirements you'll need for streaming but more cores the better, streaming requires sequential write so any SSD or HDD RAID can do it. An i7 takes 20% of your budget though. For "future proofing" i'd want to put as much into the GPU as possible, to the extent of re-using a case i already had. Without more information it's hard to recommend anything.

In terms of GPU a GTX 770 $60 more expensive according to pcpartpicker.com than a 7970 ghz edition, and a powercolour 7970 is going for $290. One could theoretically buy a more expensive motherboard & PSU, CF down the line with that. I'm going to go for a 7970 ghz as it's cheaper atm. though as it's higher power usage and less conducive to dual card later GTX 770 doesn't break your budget if you can get a case + cooler down the line

No SSD but you have to sacrifice something, removing the dual seagate 2tb drives for the space and drive throughput will get you a 240 GB SSD, i'd wait as it's not within your budget with the other components. an uncompressed 1080p stream to HDD does 211 MB/s well within the reach of an HDD raid. If you only plan on streaming and not storing as well you could definitely go SSD instead.

With a PSU i'd recommend a platinum PSU but it really depends on how much draw you think you'll have all the time, as i assume you'll be overclocknig your 4770k i assume it's a lot even at idle and i recommend a 80 plus platinum as it can save money in the long run (pay back time would be 1.6 years at the cost of electricity where i am).

This total build ends up as ~1150, i assume you can either spend $50 on a case or save and get a nice one, a cooler too.

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i7-4770K 3.5GHz Quad-Core Processor ($279.99 @ Microcenter)
Motherboard: ASRock Z87 Extreme4 ATX LGA1150 Motherboard ($139.99 @ Newegg)
Memory: Patriot Signature 8GB (2 x 4GB) DDR3-1600 Memory ($52.99 @ Newegg)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($85.98 @ Outlet PC)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive ($85.98 @ Outlet PC)
Video Card: XFX Radeon HD 7970 GHz Edition 3GB Video Card ($335.91 @ Newegg)
Power Supply: Rosewill Fortress 750W 80 PLUS Platinum Certified ATX12V / EPS12V Power Supply ($132.98 @ Newegg)
Total: $1113.82
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-16 04:48 EDT-0400)

I have no idea how this motherboard will go but it got reviewed nicely enough on Tom's earlier this month.