I actually have the system you want, and then some. i7-3930k overclocked to 4.4 ghz (Wasn't holding stable at 4.5, seems to be damaged. Don't forget acquitted cooling and run it without a fan for two weeks. <.<') with 32 gigs of RAM.
DO NOT UNDERESTIMATE THE VALUE OF A GOOD PSU! Seriously. a 750 watt PSU running at 300 watts (ANd that's mainly just the processor, not counting anything the GPU wants) is gunna heat your room really fast. I have a 1200 watt PSU that runs at room tempeture in that system. I only use 1/4th of its possible output power, but believe me you need that. It also needs to be gold rated. The better that rating, the less energy it turns into heat.
16 gigs of ram should be enough for most Lux renders. However, I have 32 gigs of RAM and 64 gigs of SWAP and actually almost ran out a few days ago when I was messing around with rainbowtables (After a while, having a system with a crapload of RAM makes you do weird stuff) -- So, just something to think about.
If you /must/ get a GPU, get a GeForce. Quadro's just have multiple outputs and are otherwise /less/ powerful. HOwever, before you waste your money, let me say something really, really, really important:
IF YOU ARE USING HYBRID RENDERING, YOUR MAXIMUM RAM PER SCENE WILL THEN BE LIMITED BY THE GPU, NOT YOUR MAIN SYSTEM RAM! Even if the devs made a patch, you'd get 2 meg/second transfer rate over PCI-e and you'd end up going slower than if you had a single core Pentium. So, I really, really, really suggest /not/ getting a pricey heat-producing GPU if you want to make really big renders or complicated scenes. I have an 8400 GT in my system and it works great.
Don't get an ATI card if you're on Linux. I shouldn't need to explain why. Also, Gentoo Linux runs Lux about 18% faster than Windows 7 Ultimate (Legit, fresh copy from a box) in my tests on the system with an i3-3930k.
Speaking of 7, you /need/ to get Ultimate, Professional or Business to use all 16 gigs of RAM. To make more money, Microsoft puts caps on the amount of RAM your system is allowed to use depending on the edition of Windows you buy. Home is 4 gigs, Home Premium is 8, Business is 16, Professional and Ultimate are uncapped.
And speaking of RAM, don't get Gskill ram just cuz everyone says to. I don't understand why people think it's the best RAM ever. It's not. It's totally average. Sure you can make the numbers go really low, but only if you set the DRAM timing to 2 instead of 1, which essentially doubles everything else. Do that and the timings are just about equal with everyone else's.
Finally, don't get a dual socket. You won't get as much benefit as you think. Overclocked, all the cores on my system add up to about 55 ghz. The fastest Xeon system out there with two sockets has 40 logical cores at roughly 3 ghz per core. That's 120 ghz and more than twice as fast as mine, BUT, each of those chips costs $4,500 alone and you can't overclock them beyond .4 ghz of their factory speed since they're locked by Intel. All Xeons are.
My processor cost $650. If you want to get a system as fast as a dual-socket Xeon (Well, just under it), just buy two of my systems. One costs $2,300 total, including all hardware. So, two systems are as fast as /one/ 20-logical-core Xeon processor, nevermind the second or the added cost of a dual-socket motherboard.
The only time I'd say to get a dual-Xeon setup is if you want a system about the same speed as mine, but you live in a really hot area. To maintain an overclock at 4.5 ghz, I had to keep the room below 70 F at all times (21.1 C) -- Depending where you live, this might not be reasonable. I had to drop it to 4.4 ghz and now it handles better up to about 80 F (26.6 C), but I've never tested it above 85-90, and I know in some parts of the world it's usual to have it 100+ every day in a row. If you don't mind spending more, you can grab two Xeon processors, not overclock them, and get the same speed but not have to worry about your ambient room tempeture. ..But then you might as well get an i7-3930k with a piezoelectric cooler, so. <.< Then again those are hard to find.
SUMMARY: Don't get an ATI GPU if you plan to render with Linux, especially cuz Linux is faster than Windows with Lux and Linux won't charge you $100 more to use all 16 gigs of RAM. You need at least a "Business" edition of Windows to use all 16 gigs, and Professional/Ultimate if you want anything higher. Don't spend too much money on a GPU even if it's by nVidia because you'll be limited by the amount of RAM the card has, /not your system/, while rendering, thus making the 16 gigs you bought totally pointless. Get a really good PSU that's gold rated since those run a crapload cooler (I'm talking going from 60-75 F (15.5 - 24 C) in an hour with a random PSU at 750 watts!), and make sure you get something strong even if you won't use it all. PSU's put out more heat than you realize. I really suggest getting a cheap nVidia card that draws power only from the PCI-e slot on the motherboard.
Oh, one other important thing. If you are getting a Sabertooth X-79 motherboard, that tiny fan above the I/O ports is REQUIRED for safe operation! It's not there to cool the I/O ports! Right under it are heatfins and they're connected via a heatpipe to the VRM's module, which, in fact, has NO HEATSINK. Without that fan you'll reach temperatures so high you'll smell smoke. The fan measures 35x35x4 and blows /into/ the case, in case you ordered one and it didn't come with the fan. (The idea is it takes cool air from the outside, blows it into the case as warm air, but blows it right next to the rear exhaust fan so it doesn't touch anything. The motherboard handles getting the fan to spin "backwords" by its self.)
Okay, two things. If you do get an i7-3930k, lemme tell you that you can stay within all of Intel's safe voltage specs and still hit 4.4 ghz, maybe 4.5. Really. If you're playing with voltages and you start seeing them change colors from white to yellow to indicate a warning, you're going too far. You really, really, don't need to go as far as people say you do with it. There's a sudden drop in performance gain right above where Intel says you should stop. (You can go from 3.2 to 4.4 ghz with a .5 Vcore bump, but to get from 4.4 to 4.7 you need another 1.5 in Vcore. Seriously.) In other words, listen to Intel. They know what they're talking about.
And don't get an AMD processor. Especially a bulldozer. Those are absolutely useless for Lux. All the cores share one single FPU, essentially making it as useful as an i3.
I will now click Submit without reading over my post and will realize later after leaving my home for a few hours that I made a horrible mistake. Have fun!
EDIT: I made a horrible mistake. I forgot to tell you that I saved links of all the stuff I had to order except a hard drive. Here'ya go! http://bitly.com/bundles/nofew/3
(If that doesn't work, tell me. Ever since bit.ly got an "upgrade" to be more social I've been having nothing but problems with them.)