Don't bother upgrading the PSU. Your 750TX will be fine, even with SLI, unless you get the absolute highest-powered cards and play games 12 hours a day. Edit: And in the extremely unlikely circumstance that you do have problems, you can always upgrade then.
You aren't likely to notice much difference with the 2600K over the 2500K; certainly not enough to justify the 50% higher price.
MB, RAM, HD all fine.
The GTX 570 and the HD 6970 are very similar in price and performance. Each has its own slight advantages. For myself I would go for the 6970 because it can run up to 6 monitors off one card. (Unlikely to make a difference to most people.) It also has a bit more memory, which helps if you're playing at very high resolutions and/or on multiple monitors. On the other hand, the GTX 570 can usually be had a bit cheaper, and IIRC works better with 3d monitors. (IIRC) You really can't go wrong either way. Another option if you want to save some cash is to flash a 6950 to a 6970, which has been reported to work very well. Of course, that will void the warranty.
I'd put your old comp up on craigslist. Even if you don't get a ton for it, best thing about selling on CL is that someone will come to your house and pick it up. Low effort = teh win.
*Shameless plug - see my sig if you want to search craigslist more easily...
I wondered about the 2500k vs the 2600k. The reason I chose the 2600k is the additional 4 threads I assumed would help with video editing. Admitably I haven't done the most research but is this not likely to be the case? How about long term, is it likely to become more of an issue with upgrades?
I saw the flash trick for 6950 to 6970 but I'd rather not void the warrenty. I'm guessing that being able to flash it up will probably not work after they apply a patch/bios update? Don't really know much about it but I'm strongly leaning towards paying a little more up front and worrying less.
I like the cooler/ram suggestion. Realistically how much of a difference would I see from the 212 to the scythe?
And I guess it can't hurt to pawn off the old computer on someone else, even if just for parts. I'm thinking maybe 20 bucks lol. Especially if it comes without any HD or PSU.
It would depend on whether the program you use for editing supports hyperthreading. I'm doubtful. Even if so though, you'd likely see ~20% improvement for 50% higher price.
When you flash the graphics card you replace the bios, so it's unlikely they could remove the ability through firmware. (The only possibility would be releasing new 6970 firmware that is somehow incompatible with the 6950. But the whole point is that they're basically the same hardware, so that's unlikely.) What is more likely is that they will make an actual hardware change, so 6950s bought in the future won't have this ability.
That said, I hear you about not wanting to void the warranty to save a few bucks. I'm planning to leave my 6950 alone, then maybe flash up sometime in the future to extend its useful life a bit. (Free upgrade! )
The Scythe is a better cooler, which should get you a bit more of an overclock. How much more I don't know. If you don't care about getting the max OC, either will work. However, the Scythe is only $10 more, and the better heatsink and better fan will also allow it to run quieter. The only real down-side is that it's larger and purportedly harder to install.