Hi,
1) Slow memory issue:
Yes, this can be an issue. It's not currently a huge issue because most games don't use more than 3.5GB (it's the last half a GB or 512MB that's slow). It's a rather complicated subject and it's not certain whether a workaround will come out such as the ability to disable the last 512MB with a driver update (and reserve it for Windows only).
So again you can get stutter in some games but it's hard to pin down the extent of the issue. It MAY be very minor.
2) DSR:
If you scale to 2560x1440 instead of 1920x1080 that's normally about a 30% performance hit (though the SMOOTHNESS factor has different options so there's a wide range of how demanding this can get).
A GTX970 will get you roughly 30% more performance.
https://www.techpowerup.com/reviews/ASUS/GTX_970_STRIX_OC/25.html
However, the GTX770 2GB will be problematic when you upscale some games as you'll run out of VRAM and massively stutter. Unfortunately by upscaling you're more prone to use more than 3.5GB so may end up with some slight stutter due to the slow memory issue in some games.
3) Would I upgrade?
NO. I would not but YOU are in a different situation (see summary). I have a GTX680 at 1200MHz so it's almost identical to your 770. I get a great gaming experience if I tweak carefully and while I need a bit more performance for games in the near future like WITCHER 3, I always hold off and wait for a card that's closer to 2X the performance of my current one.
So I'm waiting for something like a GTX980Ti 6GB card. I'll probably wait another year though and get something new or a 980Ti at a discount perhaps.
4) DSR and 2GB VRAM?
I had issues with running out of VRAM. I experimented a little bit but not too much. I also have a 2560x1440 monitor so if I don't use DSR and just apply anti-aliasing I'm pretty pleased with the results. I could try upscaling that a bit more with DSR but again I don't seem to have enough VRAM for most games.
Even TORCHLIGHT started locked to 60FPS but after a short time the frame rate plummeted because I ran out of VRAM. I forget my DSR setting though and perhaps I could have tweaked that a bit to maintain 60FPS.
Summary:
Your choice really. The GTX970 is a great card in general and many games don't really need DSR so you can play around with the optimal configuration.
Also the GTX970 currently comes with two free games so that's nice and that also means you aren't spending much extra for the 970 once you factor in your sale of the 770 and cost of games:
Witcher 3, and
Batman Arkham Knight
If you buy a GTX970 then redeem the game codes ASAP. My recommended cards are:
a) Asus DC2 version, or
b) EVGA ACX2.0 SC (1.17GHz)
You can try pcpartpicker.
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