Any of those cards will be a nice upgrade from your GT730.
Your psu can handle any modern single graphics card made today.
They are all approximately the same capability with the GTX1060 perhaps a tier higher on tom's graphics hierarchy chart:
http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
I might pass on the GTX970, mainly because it is an older gen and takes more power.
Not a problem with the mini versions. They are the same guts as the larger versions, but with lesser cooling.
Pay little attention to fancy cooling. Cooling sells, but what you get on a card will be appropriate to the card.
Open box may be just a return, or it could have a problem or missing parts.
You will get a warranty, but it is a hassle if you need it.
I do not see $200 GTX1060 cards.
I like the EVGA GTX1060SC for $210.
The SC is overclocked and stronger; EVGA has excellent customer forums and support.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487267
Considering that your FX-6300 is not a strong cpu, you might want to wait a couple of weeks for the upcoming GTX1050ti in the $150 range.
Do not use Vram as a selection criteria. My stock vram analysis:
VRAM has become a marketing issue.
My understanding is that vram is more of a performance issue than a functional issue.
A game needs to have most of the data in vram that it uses most of the time.
Somewhat like real ram.
If a game needs something not in vram, it needs to get it across the pcie boundary
hopefully from real ram and hopefully not from a hard drive.
It is not informative to know to what level the available vram is filled.
Possibly much of what is there is not needed.
What is not known is the rate of vram exchange.
Vram is managed by the Graphics card driver, and by the game. There may be differences in effectiveness between amd and nvidia cards.
And differences between games.
Here is an older performance test comparing 2gb with 4gb vram.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Spoiler... not a significant difference.
A more current set of tests shows the same results:
http://www.techspot.com/review/1114-vram-comparison-test/page5.html
And... no game maker wants to limit their market by
requiring huge amounts of vram. The vram you see will be appropriate to the particular card.