Upgrading Machine, Gaming and Moderate Engineering Use.

How much cash do you have? I am seeing a ssd raid good for this build.

Some ssds for raid: 2x 250gb http://www.newegg.com/Product/Product.aspx?Item=N82E16820147372 $249.98
Some more hdds for raid storage raid 5 4x: http://www.newegg.com/Product/Product.aspx?Item=N82E16822236404&cm_re=2tb_wd_green-_-22-236-404-_-Product $319.96

there is most of your build right there

Pcie 4.0 won't be out for a while yet it dosnt exist on motherboards never mind graphics cards and current GPUs barley saturate 2.0 slots never mind 3.0

The nas is really good idea. As for the ssds keeping it under 350 you could raid0 2x 850 pros.

Some comments:

1) GPU:
I'm not certain, but I think you can have both the Quadro and Gaming cards installed at the same time physically and swap between them in software. You definitely don't want to be physically moving them very often since it's a hassle and breaking something is very likely if done a lot.

(Ideally you would have the monitor attached to the Gaming GPU and be able to tell your video editing programs to use the Quadro from within the program settings)

Which GPU is hard to say. For the GAMING card it really comes down to budget and the games you plan to play. Without delving into all the pros and cons I'll simply recommend these:

$125 GTX750Ti 2GB http://pcpartpicker.com/part/asus-video-card-gtx750tioc2gd5 (light/medium gaming)

$200 GTX960 2GB http://pcpartpicker.com/part/evga-video-card-02gp42966kr

$232 R9-280X 3GB http://pcpartpicker.com/part/gigabyte-video-card-gvr928xoc3gdrev3

$330 GTX970 4GB http://pcpartpicker.com/part/evga-video-card-04gp42974kr

2) DDR3 difference:
Beyond 1866MHz CAS9 is very unlikely to give any real-world difference.

3) Windows 7:
*It's not transferable unless you have a FULL version which is unlikely. If it's installed on your current system it's almost certain you need to buy a new OS (Windows 8.1 64-bit OEM).

You can install W7 fine, but if it's non-transferable then Activation will fail so you can only fully use the PC for 30 days. Microsoft has a list on a server for every activated PC which specifies details about your PC especially your SOCKET TYPE. You can change a lot of things but a new socket means a new PC.

Ifyou do have W7 I'd upgrade to W10 later. I'm not certain if W7 to W10 can be done whilst keeping your progams as well. You can with W8 to W10 but then W10 is largely based on W8 code unlike W7.

4) CPU cooler:
I don't recommend liquid coolers at all. My top choice is the Noctua NH-U12S air cooler which is about $60 to $65. Why? It's silent in basic usage and very quiet with the i7 at full load.

Liquid coolers are much more prone to fail due to the pump and other things so should mainly only be used if an air cooler can't do the job. The Noctua can, is cheaper, more reliable, and quieter.

5) PCIe v4?
It's a non-issue.

*I'll make a system and put it below.

Recommended parts:
http://pcpartpicker.com/p/fQGrhM

1) The CPU has an iGPU if you don't want to buy a gaming or Quadro card right away.

2) Samsung Magician software is one reason I highly recommend SSD's. Simple to update firmware, benchmark, apply profile etc. It's unclear whether making a RAID setup will make much difference in the real world since the bottleneck when doing video editing is likely the CPU anyway. RAID0 is slightly less reliable though.

3) Don't forget to download setup fan control from motherboard site and setup to minimize noise. If possible attach case fans as well.

4) I strongly suggest Acronis True Image and setup to automatically backup your C-drive on SSD to an HDD. I have it automatically done weekly, incremental, and delete older backups.

5) With a single 250GB SSD one option is to partition in TWO with Windows on one half and use the second half as a temporary work space for video files. Then, store the video on your HDD and copy over when you wish to edit.

125GB (before formatting) should be well over what you need for basic Windows, programs etc but for GAMING I'd strongly suggest installing those on your HDD such as:
a) Steam-> install to C-drive but create a Steam folder on your HDD (E drive?) and install games to there not C-drive. It's very easy to do that.

"E:\Steam2"

b) Other games-> "E:\Games"

Cheers.

OTHER:
If you have STEAM games already you can move them to the new machine rather than re-download. Simply install Steam and I think you can just copy over the Steamapps folder. I did it myself and it worked fine. SAVE GAMES are mostly in your "Documents" folder so you'd need to find and copy those unless the game has saves on your Steam cloud (start game first to see).

If using the 2nd Steam folder as I suggest you need to create that folder to have a Steamapps folder to copy the files to. That's located at:

Steam-> Settings-> Downloads-> Steam Library Folders-> "Add Library Folder"

Alternatively you can make game BACKUPS and RESTORE them to the new machine (backup a game, copy game backup to new machine and choose restore in Steam and find the game to restore). I can't think of a reason to use this method over copying the games in the Steamapps folder though.