Looking to upgrade,need some help

1) It depends on your games and your monitor. If you are doing 1080p 144Hz, or 1440p 60Hz you will definitely see a benefit with the 1070. Otherwise it's a bit overkill. If you are only playing CS:GO, LOL, and other light usage games, it's overkill.
2) Depends, see above.
3) Great processor. I don't see it as overkill for current games and applications. But your current processor is still good too. If it were my money, I'd stick with the 3470.
4) If you are moving to Skylake, yes.

Why do you want to upgrade?
In what way is your current pc not doing the job?
To answer your questions:

1. Worth is something only YOU can determine. Generally, a GTX970 class graphics card will run most games well at 1080P.
GTX1070 is more than you need for that. The GTX1060 is supposed to be a GTX980 class card and would seem to be a good upgrade for 1080P.

2. Pick a EVGA card. They have good warranty and support. Plus, a 90 day free upgrade option lets you be certain you bought a card strong enough.

3. I would not buy I5-6600. Not because it is not good, but because for about 10% more $ you can get some 40% more compute power with a I5-6600K that can be overclocked.

4. If you buy skylake, as I think you should, you will want DDR4 ram.
DDR3 ram only works if it is low voltage(1.35v) DDR4 ram is no more expensive than DDR3 anyway.

If your games are strategy, sims, mmo, you will want to look more at cpu single thread performance improvements.
If your games are fast action shooters, look more to graphics. I would think your I5-3470 might do well enough for those.

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To help clarify your CPU/GPU options, run these two tests:

a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.

b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.

You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. You will need to reboot for the change to take effect. Set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.

If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.

It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
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The psu you need will be determined by your graphics card.
Here is a chart:
http://www.realhardtechx.com/index_archivos/Page362.htm

Buy a tier 1 or 2 psu. DO NOT buy a cheap psu.
A cheap psu can become very expensive if it fails under load and damages your other parts.
Here is a chart:
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html

Could you link me to the benchmarks that show a 40% increase? Everything I've seen shows a 1%-5% increase in gaming performance between a locked i5 and a 4.5Ghz i5.