(New to forums, but I am very active at OCN and a long-time reader here so I figured it was time to join; HI!!! )
Personally, if I were in your position, I would very much consider turning the i7-920 rig into a secondary rig for use as a Home Server/Folder(F@H/BOINC/etc)/Kick@ss-HTPC all-in-one, and get a 3930K to build around.
I recently did my first build in a long time, well, my first build for myself. I went with the 3930K and Rampage IV Extreme because I was at Micro Center and it must have been a lucky day because: the chip was $489 "on sale" from $569, the R4E was accidentally mislabeled as "open box" although it was factory sealed (confirmed with Asus) yet Micro Center honored the labeled price so $319 for the board. I signed up for the MC CC which gave me 12mo no interest, I had a voucher for "15% off ANY purchase" that I was given due to having an HDD fail that I bought from them within 5 days and they were unable to get me the same model for replacement so they gave me the discount "for being a loyal customer and to compensate for the loss of time and possibly work", AND they were running two concurrent deals in which I got an extra $75 off for buying an "Intel 3xxx CPU with a Series-7 Motherboard" (which was advertised for IVB/Z77 but I pointed out that the 3930K was indeed a "3xxx" processor and the R4E was indeed a "series-7 motherboard; X79 = series 7"; the manager said that I was correct, and they would honor the advertised promotion), and a free 64GB SSD (one of the "Cache" SSD's) "with purchase of a 6-Core SandyBridge-E Processor"! That's $858.65 (with tax), ~$740 after 15% discount for signing up for their CC (which requires at least $500/mo spent on it, but I spend it anyways and I am able to transfer purchases on MC CC to Amazon CC for the Amazon Prime points), and about $670 TOTAL OUT THE DOOR. Keep in mind that includes the 3930K, Rampage IV Extreme, AND a FREE 64GB SATA3 SSD! At that point, because of the stars aligning, I was able to get out of there with the highest-end X79 board, a 3930K (less than 1mo from Mfgr date of chip), and a solid state drive all for the price of a 3770K + upper-mid-range Z77 board.
I had waited many months for Ivy Bridge to release, and real world data to pour in, before deciding which way to go with my new build. Originally, I had planned on a 3770K + Maximus V Extreme and a beautiful customized CaseLabs TH10 case in White (85mm Extended Vent Top, Magnum Pedestal, 4x Flex Bays and 4x regular, 4x XXL Accessory Bars, and a mix of various 120/140 fan mounts), and I had high hopes for IVB. Even after the initial reports of "OMGWTFBBQ My chip is running two-point-five bazillion degrees at just 4.5Ghz!" and eventual discovery of the TIM being to blame, I was still hopeful that it could be fixed and was just waiting on the Maximus V. After a few months post-release, I couldn't stand it anymore and realized that, as I don't do "full upgrades/overhauls" that often, maybe every 4 years, I would always regret buying a "dead" socket and a CPU that has serious OC limits for daily use.
The more and more I read about the SNB-E chips, specifically the 3930K, as well as the entire LGA2011 platform, the more interested and intrigued I became. While I was initially scared away due to the cost (which CAN be substantial), I began to realize that 2011 is truly the successor to 1366, which was perhaps the absolute best platform up until 2011 succeeded; in fact, so many people run a 3.5yr old socket that it is anything but dead!
Knowing that I would be adding a second, and third, GTX670FTW 2GB to the system to help with rendering as well as gaming on my pre-ordered Dell U2713AH (1440p AH-IPS), would likely be utilizing a Hardware RAID Card, and would definitely be using one of my professional-level audio cards....The 40 native PCIe3.0 lanes held a huge advantage over 1155 and with the latter I would be sacrificing either GPU power or up-to-24x 2TB drive RAID array.
The 8x DIMM slots was something that was appealing to me as well, not because quad-channel memory is much of a boost in performance (it's not) but because I would never, during the entire projected life of the board, have a need for more memory than its 64GB maximum.
Lastly, the 3930K is the "GOLDEN CPU" in my opinion, as it has 95-100% of the 3960X which is twice the price, a solid 12MB L3 cache vs 8MB MAX for IVB, SIX CORES with 12 threads giving it unprecedented amounts of power in any situation you put it in (gaming performance is equivalent to the lesser-priced CPU's, but workstation performance is not even in the same city, much less ballpark).
Looking at your system in your profile, it would seem like simply switching to an X79 Motherboard and a 3930K would give you significant gains beyond a 990X without changing anything else, although now that AMD 6000-series prices are falling especially if you grab one off a forum's classifieds section, it would certainly be a big boost to grab a second 6950 2GB or even a 6970 2GB (depending on whether yours unlocks). 3930K + R4E/Extreme9/P9X79-Pro/X79-UP5/etc + 2x 6950 2GB/6970 2GB in CF-X would be a wicked combination, especially with the dedicated PhysX card you have.
Your RAM would work simply by dropping it in, as the platform supports dual, triple, AND quad-channel memory and thus you would have no need to upgrade the memory until you feel it is prudent to do so. I will say that running 16GB (4x4) of G.Skill Ripjaws-Z DDR3-2133CL9 @ 2520 10-12-11-30 or 2260 9-10-10-28 in quad-channel gives insane amounts of throughput!
Bottom line: if you are looking for another "wunder-chip" akin to your 920 in terms of performance, bang-for-buck, and longevity there is no better option than a 3930K with a good board; just make sure that whatever board you may choose has: all 8 DIMMs for future expansion, supports x16-x16-x8/x16-x8-x8-x8 3-/4-way SLI/CF-X, has better than average cooling for the chipset and especially VRM/MOSFETs as they definitely get hot when overclocked, and one that is known to support PCIe3.0 with little or no issue.
Best of luck with your upgrade!