best new rig specs

There's nothing wrong with Windows 10. You get it now it's not going to cost you any monthly fees. 

It will be best for gaming.

win 10 won't be a monthly subscription... for home users.. but it'll suck if you are on seriously low end hardware as they'll probably not be supported at some point. (like some right now)

https://arstechnica.co.uk/information-technology/2017/07/windows-10-support-device-lifetime/

http://wccftech.com/microsoft-ends-windows-10-support-intel/

shouldn't think it'll affect mainstream desktop users any time soon

Honestly it's hard to give a true opinion on what to buy for best bang for your buck, can you give us a bit more detail please?  IE what version of Sabertooth board are you running?  This will tell us what Processors and Ram types you can handle...

For the games you have listed & assuming future gaming demand...

I'd suggest

High quality build 750w+ Gold/Platinum or higher rated efficiency - Troubleshooting faulty power is a pain

Modern Motherboard to handle the new CPU - I personally would suggest one with a PCIe Gen3 M.2 socket that supports NVMe booting

A modern Core i5 or AMD Equivalent - Note as of Intel 7th Gen Microsoft only supports those processors on Windows 10

16GB of quality RAM - I've never really been one to hard push those with tons of OC capabilities or LEDs

Nvidia GTX 1060 6GB Ram card or ATI Equivalent

M.2 NVMe Solid State for OS & most important games

Recycle any old components possible, and replace the rest as needed / desired.

Also as you see I didn't post specific part numbers -- What works for me may not work for you.  So yup Adult Lego time

Best Buy has a gaming desk top with an I7 processor and Nvidia's new 1080ti card for $1600 

The 1080ti has 11gigs of on board mem 

I have a slightly more powerful rig that turn times for largest GCIII map and 8 opps is in the seconds.

Invest in a good Solid State Drive as well. They come in many shapes and sizes now that the market is saturated with them. I plan on getting a 500gb m.2 SSD for my next build. I've been using a SSD for my operating system drive for years now and its amazing. 

Link to the build I'm doing in a month or so.

 Price wise i recommend starting with asus.

LOL depends on what you have in mind Admiral. 

My RIG: https://onedrive.live.com/?id=7D34673FE36FD740%2151262&cid=7D34673FE36FD740

Chassis: Cougar Conquer open frame

MB: ASUS ROG Strix X299-E Gaming

CPU: Intel Core i9-7900X Skylake-X 10-Core Socket 2066

Ram: G.Skill Trident Z RGB LED DDR4 PC33000/4133MHz 32GB

Storage: Samsung 960 PRO 2TB

PSU: EVGA SuperNOVA T2 1000W Platinum + Carbon CableMod

GFX: SLI ROG-STRIX-GTX1080TI-O11G-GAMING + HB ROG SLI Bridge

Cooling:

Rad: Alphacool NexXxos Monsta 360 + 3 Corsair ML120 PRO LED

Pump & Res: EK-XRES 140 DDC 3.2 PWM + 2 Cougar Vortex CF-V12HP PWM 120mm fans

CPU: EK-Supremacy EVO (Original CSQ)

Tubing: EK-HD PETG Tube 12/16mm

Monitor:

ROG SWIFT PG348Q

Pheripherials:

Keyboard: ASUS ROG Claymore with Num Pad (MX RED)

Mouse: ASUS ROG Gladius II

Pad: ASUS ROG Sheath

Audio:

Creative Sound Blaster X Katana (Unbelievably awesome).

ASUS ROG Centurion 7.1

whatever you do, you're going to need to get a new motherboard which will cost some bucks.  I would recommend staying intel.  For high settings in witcher 3, you want better than a GTX 970 (that nvidia hairworks drops it to below 60 fps).  If you want to break-the-bank bad graphics upgrade (and guarantee oldrim will run at max settings), you need a 1080 ti.  The card WILL set you back almost $800 depending on where you buy it from and what manufacturer, over half your budget.  With that card, I would echo what others say, get a platinum rated PSU (I have a Corsair HX860i (860 watts and modular) which was a RMA upgrade from a HX 850i (modular 850 watt PSU).  Both platinum rated.  Former is a replacement due to a outta spec 3.3 rail which caused ASUS anti-surge reboots.  Cost for the express RMA was $230 to give an idea of PSU costs.).  Fifth gen intel (socket 2011v3) should be able to be on win 7 if it's still being sold.  Maybe 6th gen.  7th is a possibility.  I could give suggestions but you'd be breaking the bank.  Re-use your optical drives if they are recent enough, get a new case, get aircooling (aftermarket fans will work).  Get a Noctura heatsink (probably as good as watercooling).

my PC specs (built in 2015)

CPU: intel i7-5930k

heatsink: Noctura NH-D15s

motherboard: Asus X-99 deluxe w/u3.1

RAM: 32 GB DDR4 RAM (forget who made it off top of my head)

storage: you don't want to know but it's a lot (2 TB SSHDD partitioned to win 7 and win 10 boot partitions if you need to know)

optical: Pioneer BD-XL drive and older Pioneer BD-burner

case: Rosewill rise (do NOT get this case as the third drive bay is mounted on top and makes it dangerous to put a drive there; I broke the SATA data cable on a drive moving a PSU around after mounting it wrong.  The case has a bottom vent that is not ideal for a large PSU.  That bad mounting was responsible in part for my first RMA.  This is my second.)  I would go with a NZXT case.

cooling:

3x120 mm fans for front aftermarket.  1x 140mm fan top that needs replacing.  1x 80 mm fan covering the 2nd PSU slot.

monitor: Asus VG248QE 3d ready monitor

sound: onboard 7.1 but hooked up to a 7.1 headset bought this year.

GPU: EVGA GTX 1080 ti FTW 3 GAMING ($813 after taxes FYI)

With the current fan configuration, my CPU can get package temps of 47 celsius (each core sub 39).  Ram hits about 39.  GPU hits 36-37 with precision xoc off (much lower on).

I've been away for a year.  Finally back on line.  Retired - LOL. Medicare - oye.  Anyway - my old rig is maxed out.    Still can't run Witcher 3.  Or oldrim... very well, or skyrim VR.   So, looking at new PC.   Research - its all very complicated now.   (Or I've been out of the loop too long.)   I'll also want to play the rainbow (Clancy) games.  Never have - but the vids look good.  

Second use:  Doing small video editing.  I've written a few screenplays = but the way to get connected - it seems -is to have a 'social media' presence - and showcase your shorts (videos) to attract interested parties to greenlight the serious screenplays (meaning feature length - and/or episodic).   My way of giving back to the world.   I'm thinking that graphics are graphics - run a current game well, and the vids should be fine, as well.   So, what do the experts suggest?