Just changed my GTX580 for a HD7970.
GTX6XX are just a joke (as is the driver BS we read everywhere. I did GTX275, HD5870, GTX580, HD7970, drivers are the same. Small problems both sides, nothing important both sides)
In france :
GTX680 : 450€, 81%
HD7970GHz : 400€, 100% (yup, AMD respects dollar/euro parity. nVidia ... well, greedy dude is greedy)
Titan : 1000€, 112 (118% watercooled or with additional FANS).
(please don't respond to this with all the figures from sites who didn't test the titan once warmed up and at realistic frequency, or on bench table ...)
Now, if you actually want the GTX780 to be a 750€ equivalent of what should have been the 750Ti (as GTX680 is really a 450€ 660Ti+ in the first place) :
Don't worry about PCIE2.0/3.0, won't change a thing. Actually PCIE3 has to be registry activated on some platforms, and changes only performances for 2/3 cards SLI setups.
Also the 680 is quite gentle on PSUs actually, unlike the 580. It will eat up to 189W in games (better than 230W monsters like 580 or 7970), and even 500W shall be enough if it's good quality. If you have two lines of 16A each, it's not enough I think. Get an Antec HCG or something like that.
I'd stay clear from the 680. The 670 is 100€ less for 5% performance down. Depending on resolution, I'd stick to 670 over 660Ti (192 bits BUS, bad for future/High res/MSAA). If you're 1680*1050 or 1920*1080 no AA, then 660Ti is fine I guess.
Finally, modern cards tend to have coil whine. This should not be accepted as a fact if it's very strong, return the card.
As for brands, my advices are Gigabyte Windforce models, or ASUS DCU2 (my 580 was this model) if you want one inch better for two inches more expensive.
R9 3900X/X470 Taichi/32GB 3600CL15/5700XT AE/Marantz PM7005
i7 980X/R9 290X/X-Fi titanium | FX-57/X1950XTX/Audigy 2ZS
Athlon 1000T Slot A/GeForce 3/AWE64G | K5 PR 200/ET6000/AWE32
Ppro 200 1M/Voodoo 3 2000/AWE 32 | iDX4 100/S3 864 VLB/SB16