Typically costs of CPU revolve around cache per core and speed of that core and quantity made/ available.
I think the Extreme line is over priced too, but if you have program that will work with 4 cores and you need to be more productive.. get the CPU with more cache, speed, and HT.
Regarding the Xeon Quad Core: I think it is bus speed, and Cache that make it attractive. I don't think it was made for render speed.
Would be interesting to put them side by side (near equal costing CPUs) and see which one is faster for video rendering.
Some Gigabyte MOBOs have 1394b ports (firewire 800)