I'm thinking of getting a 6670 just for rendering videos because Sony Vegas doesn't really support 7xxx series cards. Would it work?
Don't bother going down a series in cards. If you really want horsepower just go over to nVidia. From what it sounds like you're wondering whether or not you can use the 6670 for just rendering and have a 7xxx card for everything else. That won't work either due to the cards incompatiability with crossfire. I would suggest a good GTX 760 or 770. You can also wait for Sony to get it together with 7xxx support.
what do 760s compare to in AMD/GTX 6xx series cards?
what do 760s compare to in AMD/GTX 6xx series cards?
Vegas supports the 7xxx series of cards just perfectly fine.
The 760 compares somewhere within the 69xx series
No point geting a 760 over a 660ti the 760 has less cuda cores than the 660TI (if thats what your both thinking about becase AMD cards will destroy nivida cards useing open CL vs cuda)
I saw this http://forums.creativecow.net/thread/24/964607 and it said that Vegas falls back to CPU with 78xx cards. I think I'll keep my graphics card and try and work out a way or wait for Sony to do something,
You could grab a HD6970 if Vegas only supports 6xxx card's there around $150 on newegg or £160 in the UK.
What would a 6970 compare to?
http://www.tomshardware.com/reviews/geforce-gtx-660-ti-benchmark-review,3279-12.html
Open CL benchmarkings.
What is it like for gaming?
for gameing its on par with a HD7850 http://www.anandtech.com/bench/Product/509?vs=549
Should I just keep my 7870 XT and wait for Vegas or something to improve? Or overclock my CPU?
If you allready have a 7870XT you whant to get softwhere that uses it like adobe it's going to be cheaper than buying a new video card.
See, I tried using Premiere Pro and it rendered with CPU aswell when I told it to use GPU... I don't even know why! I did an OpenCL benchmark and it performed well so I don't really understand the problem.