I am also planning to get another 7870 to the one I already have for Crossfire and a 750W psu for the extra OC headroom and other nicknacks like future water cooling.
But what I really want to discuss is whether SB or IB is better and do I need the extra HT?
-My main purpose is for gaming.
-I OC and want the best possible performance per clock there is.
-I want to start streaming, h.264 encoding, video editing and will the x7xxk be better over the x5xxk as I will only probably start with streaming?
-What am I losing out if I got with SB over IB and vice versa?
Hyperthreading if you're only gaming is basically worthless. Simple answer go with the 3570k. Why? Better price for the performance. While it's a 5-10% increase to go with the 3770k, it's a 30-40% price increase. (Ivy bridge CPU's are 5-10% better than their predecessors)
If you are going to be dedicated to video editing/livestreaming, hyperthreading will help out a lot.
Hyperthreading is worthless, and can, in fact, hurt performance in some games. But as of the moment, basically no game benefits from it, period.
The sandy bridge chips CAN overclock "higher" (to a higher number), but ivy bridge is faster at any given speed, so seeing as it's a wash, it's better to go with the newer tech which is more efficient and has more features.
As for streaming, no, having hyperthreading won't help much with that. (If you were very very serious about it, a 6-core Xeon would be the way to go, as it's 6 physical cores, not logical ones.)
The only thing you lose out with by not getting Sandy Bridge is that, when you overclock, it won't be as high a number before the "GHz." (But that's only good to impress computer-illiterate people who think they know computers.)
As for crossfire of two 7870s, a 750w PSU, provided that it's reliable, is MORE than plenty - it's overkill for a pair of 670s, or possibly even 570s.
While Hyper Threading (HT) is useless for gaming, it is very useful for video encoding and editing. However, HT does cause an average of a 2% performance drop in games.
Ivy Bridge CPUs are on average 6% more powerful than Sandy Bridge at the same clock speed. They also draw a little less power. The problem is heat dissipation. Ivy Bridge CPUs runs hotter than Sandy Bridge because it does not dissipate heat as well. Therefore the overclocking potential is decreased. But the average of 6% increase in performance can help to make up the difference. An Ivy Bridge OC'ed to 4.4GHz is equivalent to a Sandy Bridge OC'ed to 4.66GHz.
Quick Sync video quality has been improved in the graphics core. Quick Sync is a video encoding tech developed by Intel which can dramatically decrease the time it takes to encode a video. I would say it is about 2x - 4x faster than just relying on the CPU alone and it is faster than nvidia's CUDA and ATI's OpenCL; depending on the circumstances. However, you need to use a video encoding program that can make use of Quick Sync. Arcsoft's MediaConverter and Cyberlink's Media Espresso can make use of it.