1) GTX1060 6GB - $320 (for one of the best cards)
http://pcpartpicker.com/product/3bL7YJ/asus-geforce-gtx-1060-6gb-strix-video-card-rog-strix-gtx1060-o6g-gaming
2) GTX1070 8GB - $415
http://pcpartpicker.com/product/Tv38TW/evga-geforce-gtx-1070-8gb-sc-gaming-acx-30-video-card-08g-p4-6173-kr
*One of the good GTX1070's for about the $420 to $450USD price range is by far my best advice.
3) GTX1070 PERFORMANCE->
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_1070/24.html
4) GTX1070 vs HD6850?
It's hard to compare because you only have 1GB of video memory which very few games work with at the HIGHER graphical settings so they can really tank the frame rate. If there was no VRAM issue then your FPS is probably about 5X as high.
So 50FPS instead of 10FPS.
It's going to vary a lot for other reasons, but basically it's a massive step up.
4) System memory?
My board only supports 4GB per stick, so I have a 4x4GB kit. You should also buy IDENTICAL memory modules, preferably in the same kit. If you can find the EXACT same kit it should work fine.
For example, if you have 2x4GB kit and two spare slots, find the EXACT same kit and install to the remaining slots.
Or buy a 4x4GB kit like this:
http://pcpartpicker.com/product/rxzv6h/gskill-memory-f317000cl9q16gbzh
5) Installation.
I would do this->
a) download the drivers for the card from NVidia (manually)
http://www.geforce.com/drivers
b) download and run DDU to remove your current drivers (use recommended option for safe boot)
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html
c) after it reboots, SHUT DOWN again
d) Swap video cards, ensure card is fully seated and power cables attached etc.
e) monitor attached to new card
f) startup
g) Install the NVidia drivers you had downloaded
h) go to desktop-> right-click then find the display settings and configure resolution, as well as DPI scaling if necessary (I use 125% for a 1440p monitor)
Other:
- possibly GSYNC monitor at some point but they are expensive
- setup proper ASPECT scaling if not done (I don't use the monitor itself. I use the NVidia Control Panel and set to "ASPECT" and to scale on the "GPU")
- use Adaptive VSYNC for some games if applicable (toggles VSYNC on and OFF. I won't go into detail here)
- NVidia requires a sign in now for the Geforce Experience software (which includes NVidia Share to record gameplay).
Summary:
- GTX1070 recommended
- learn NVidia software tweaks/setup
- 16GB system memory? (no rush, though memory prices for DDR3 may go up)
Finally, if you add a GTX1070 I think you should get about FIVE YEARS or so out of this computer. It's hard to say how CPU requirements will change because DX12/Vulkan, when properly optimized, can make use of most of your CPU while using more efficient coding.
So the i5-3570K should last, even when driving a good GPU (more draw calls etc).
DX12/Vulkan games could start adding code that uses the extra threads now that multi-threading is starting to work well, but these changes tend to happen over YEARS. They are also influenced heavily bu the amount of processing power the game consoles work.
Anyway, you can always revisit this issue in a few years or if your motherboard or CPU crap out.