The i7 is "better" but it's significantly more and usually not worth it for a gaming build. The i7 would come in handy of you plan on streaming or doing video editing, or something similar.
No I would not consider using two gpus at once. Always stick to the best single gpu you can afford.
Yes this system would be very easy to upgrade. The i5 would last years and years, and the video card is very quick and easy to upgrade.
I wouldn't spend anymore than that build above until you know exactly what you'll be doing with the PC and what monitor you plan on having. Then you can choose which parts would benefit from upgrading.
If this is just a gaming build, and won't be used for much else, jumping up to a GTX1070 makes the most sense. Note: this is overkill for a 60hz 1080p monitor. I recommend the 1070 for a 144hz 1080p monitor or 1440p.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-7500 3.4GHz Quad-Core Processor ($198.99 @ SuperBiiz)
Motherboard: Gigabyte GA-B250M-DS3H Micro ATX LGA1151 Motherboard ($73.98 @ Newegg)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-2400 Memory ($89.89 @ Newegg Marketplace)
Storage: Crucial MX300 275GB 2.5" Solid State Drive ($85.59 @ B&H)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.55 @ OutletPC)
Video Card: EVGA GeForce GTX 1070 8GB SC GAMING ACX 3.0 Black Edition Video Card ($376.99 @ SuperBiiz)
Case: Cooler Master N200 MicroATX Mini Tower Case ($41.99 @ NCIX US)
Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply ($69.99 @ Newegg)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Total: $1075.55
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-01-22 11:47 EST-0500
For there, if you still want to spend more money, you could either drop an i7 in there or step up to overclocking possibilities.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i7-7700 3.6GHz Quad-Core Processor ($309.69 @ SuperBiiz)
Motherboard: Gigabyte GA-B250M-DS3H Micro ATX LGA1151 Motherboard ($73.98 @ Newegg)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-2400 Memory ($89.89 @ Newegg Marketplace)
Storage: Crucial MX300 275GB 2.5" Solid State Drive ($85.59 @ B&H)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.55 @ OutletPC)
Video Card: EVGA GeForce GTX 1070 8GB SC GAMING ACX 3.0 Black Edition Video Card ($376.99 @ SuperBiiz)
Case: Cooler Master N200 MicroATX Mini Tower Case ($41.99 @ NCIX US)
Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply ($69.99 @ Newegg)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Total: $1186.25
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-01-22 11:49 EST-0500
or
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-7600K 3.8GHz Quad-Core Processor ($239.99 @ SuperBiiz)
CPU Cooler: CRYORIG H5 Ultimate 76.0 CFM CPU Cooler ($56.69 @ Amazon)
Motherboard: Asus PRIME Z270-A ATX LGA1151 Motherboard ($166.99 @ SuperBiiz)
Memory: G.Skill Ripjaws V Series 16GB (2 x 8GB) DDR4-3000 Memory ($92.99 @ Newegg)
Storage: Crucial MX300 275GB 2.5" Solid State Drive ($85.59 @ B&H)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.55 @ OutletPC)
Video Card: EVGA GeForce GTX 1070 8GB SC GAMING ACX 3.0 Black Edition Video Card ($376.99 @ SuperBiiz)
Case: NZXT S340 (Black) ATX Mid Tower Case ($62.99 @ SuperBiiz)
Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply ($69.99 @ Newegg)
Operating System: Microsoft Windows 10 Home OEM 64-bit ($88.58 @ OutletPC)
Total: $1290.35
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-01-22 11:51 EST-0500
Note: overclocking requires a better more expensive motherboard and a nice cpu cooler. I also upgraded to faster RAM and a bigger case.
From here, if you're going to be using a nice 1440p or 4k monitor you could upgrade to a GTX1080, or get a bigger ssd, or different more expensive case, etc...