WARNING: Long winded content - TL;DR section towards the bottom.
Well I have a build going on 3 years, which is running a 4x 2GB, and I see little reason to upgrade beyond 8GB. 32bit applications still have a dominance (even in gaming), and with that, they do have a RAM limit, which is less than 4GB. While 64bit applications are becoming stronger, it will take a while for them to overtake 32bit applications, so 8GB will be plenty for a long while. While I would encourage dual-channel just out of tradition (and starting building in the DDR2 days, where dual-channel was a big thing), I don't see a whole heck of a lot point to it now since there are very few applications that are held back by memory bandwidth. The most common one is the use of IGP (Integrated graphics), such as AMD's APU's, but that's irrelevant to you since you're using a 780 (Ti).
While the TechNoob does bring up a point about multi-rail designs often being a sign of lower quality, it isn't necessarily true, and EVGA does back that up with a 10 year warranty (otherwise EVGA would be loosing a lot of money in warranty replacements). I think the Super-Nova might be a good product, but I opted for the Rosewill HIVE because it is more in-line with your build's power consumption (ie not overkill), and is cheaper. Another thing to note about power supplies is who actually makes them. A lot of companies will take an already existing design, slap their label on it, and resell it. Seasonic is a company that designs and manufactures their own power supplies, and often manufactures for other companies as well. XFX has their power supplies designed and manufactured by Seasonic. That EVGA SuperNova you pointed out was likely designed and manufactured by FSP. Corsair designs their own power supplies, but has them manufactured by another manufacturer. In the case of Corsair's Builder series (CX), they have them manufactured by Channel Well, and I don't really trust them as they make power supplies for a lot "generic" (synonymous with garbage) companies. I've also received a DOA CX430, and the wiring was really short, indicating some corners were cut. On an in-depth review, there was a comment about less than stellar ripple noise. So, Corsair's Builder series are not my go-to, despite their amazing value. Seasonic makes a lot of Corsair's higher end power supplies, so that's why a lot of people recommend Corsair. So to that comment about Seasonic, XFX, or Corsair, that translates to Seasonic, Seasonic, or Seasonic. In the case of the Rosewill HIVE series, they are designed and manufactured by Sirtec, which I believe to be a decent manufacturer, and that particular power supply has recieved lots of positive reviews on both Newegg and Amazon. You can find out more about who's who in Power Supplies here
Depending on how many console ports you play, and how determined Sony and MS are to drag out this generation as long as possible... FOREVER. Joking aside (sort of), the 780-Ti is, for the most part, overkill on 1080p gaming, so as long as you have 1 1080p monitor, with no plans to upgrade to 4k, you should be set for at least a couple years without having to turn down settings. It even keeps up with Crysis 3 maxed out well above 40FPS, which is notorious benchmarking game, often leaving future generations of GPU's struggling. Case and point: Crysis 1, a game meant to punish your 8800GT, is still a handful for a 680, which I guess is now a 770.
If you happen to get a G-sync monitor, you can prolong the life even more. However, a GPU upgrade, which is often simple, is an affordable upgrade to prolong the life of your system. My personal build (which is about 3 years old now), originally had a 560-Ti, and after a series of dead GPUs and trades, ultimately lead me to a 650. I upgraded to a 760, and now my system is still playing games with very high settings with no issue, running stock clocks, though I did have my 2500k OC'd to 4.5GHz at one point. So, if you really want to be smart with your money down the road, keep that Xeon build, but downgrade the GPU to a high-end value leader like the 760, upgrade it when the need arises (I suspect games will still be heavily GPU bound for the foreseeable future), and the GPU upgrade will breathe new gaming prowess into your aging system, as it did mine. Besides, CPU upgrades lately have been incremental, almost plateauing; instead of pushing the performance envelope (as we're quickly tapping out silicon for microprocessing), Intel and AMD are now pushing more towards mobile technology, integrating more systems into the CPU (GPU being a big one recently, memory controller a few years ago; becoming more SoC-like) and increasing efficiency. So that CPU should last you a very long time.
TL;DR
Games probably won't use more than 4GB in the foreseeable future due to 32bit processing dominance, it will take some time before 64bit processing takes over. As a result, 8GB should be plenty for gaming for a long time to come.
The PSU you picked is good, but not needed. The HIVE is also good, and better suited for your build. Seasonic designs and manufactures most of the highly recommended power supplies, including from XFX and Corsair.
I suspect a 780-Ti will last a while, but if you want to be smarter with your money in the long term, get a 760 and upgrade the GPu a few years down the road. CPU progress has been slow of late (performance wise), so that CPU should last a lot longer than the GPU.
