My observations on your questions.....
I, too, was running an i7 920 at 4.2GHz or so and upgraded to a SB 2600k on an Asus Maximus IV Gene-Z.
Do I regret the upgrade? Not one iota. The 920 ran much hotter and was much trickier to achieve "good" OC's vs the 2600k cpu. The 920 was watercooled to maintain decent temps at 4.2GHz and beyond; the 2600k's OC to 4.6GHz was almost a no-brainer and could do it on decent air cooling, although the watercooling setup I had transferred from the 920 to the 2600k.
Was I worried or concerned about the USB 3.0 and PICe 3 as they existed on the Gene-Z vs. what will appear on the IvB specific chipsets? Not in the least. The Z68 has USB 3.0 and the lack of PCIe 3 isn't a concern as current gen video cards don't saturate the PCIe 2 bus at all. Will new gen. video cards finally saturate that bus? Maybe, but it's not something I got my panties into a wad about....
Is it cost effective? Only you can answer that, but in my case, when I sold my 920 cpu and the Asus Rampage III Gene motherboard it was installed on to upgrade, I realized almost enough out of the sale to buy my 2600k and Maximus IV Gene-Z motherboard with little additional cash outlay. (In fact, the guy who bought my mb and cpu wanted a complete system built around them, which I did for him. After all was said, what he paid me for the old cpu, mb, ram, along with new case, ps, optical, hd, and assembly gave me a net profit, albeit a very small one.)
Now that SB-E has debuted, the 920 and its S1366 motherboard are daily becoming worth less and less. Every day you wait to sell the set, they're worth less and less. My take was to upgrade while the old 920 had its highest "used" price.
In all, I'd really recommend upgrading when you want. Old parts do decrease in value over time, so the sooner one unloads old parts, the more they're worth. And you can wait and wait for the next greatest feature to appear on the next gen. of chipsets/cpus, but then you'll be waiting a long time.....new stuff always brings with it new features, etc.....and all the while you're debating whether it's worthwhile to upgrade or not, those of us that have are enjoying our faster machines.
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