Okay. Hi. I have done this. What I learned is:
Firstly, know your needs. It's ok to exceed them if you think you'll need the extra horsepower at some point before your next build or purchase.
Second, keep in mind that most audio processes don't require much. Some do, and it depends on how you like to work. If you require 3+ displays with heavy plugins (some metering plugs, believe it or not, are quite greedy; convolution is pretty heavy, 4x oversampling multiband comps at high sample rates, etc) running at all times, you'll need more than if you're just laying tracks, programming synths, and bouncing out. Again, know your needs.
Now for the good stuff...
More CPU cores are paramount to high performance audio workstations. You may not need 12 cores, but more is definitely better. It is far more important than clock speed, but having more GHz certainly has a significant impact as well.
You don't have to get a xeon, but my recommendation lies with any of the X99 consumer chips. Your budget may dictate this one. Do yourself a favor- if you're on the fence about a fast 4 core vs a slower 6 core, get the 6 core CPU.
Another thing to note is having a CPU that has more power than you need will run much quieter than a cheaper one you're pushing hard. Since you didn't mention having a server room for your computer, I'll assume it's in a tower near your desk.
RAM does not need to be ECC, but it doesn't hurt either.
Memory speed matters, but not as much as capacity.
If you're running several VI's, get yourself 32GB and you'll be good to go. On X99, 4x8GB sticks are more likely to work than 8x4GB sticks, so I've heard. It seems also to be the case with my rig (5960X and Asus X99-E WS).
As for the RAM speed, it has quite a large impact on performance, but prioritize capacity and CPU if your budget constrains you. I ran the Dverb test several times in PT11 with some different configurations; here are some of the results (I believe that at the time, the CPU was at 4.2GHz on 8 cores):
32GB 3000 MHz - 1340 Dverbs
32GB 2250 MHz - 1100 Dverbs
32GB 1333 MHz - 945 Dverbs
CL timings were the same for all of them.
Obviously, running over a thousand reverb plugins at once is completely ridiculous, but it shows that while memory bandwidth has little use in gaming, it is important for a DAW. VI's and other plugs can utilize that bandwidth just as much as Dverb can.
One more thing on memory- I used G.Skill memory for a year before switching to the Kingston stuff (HyperX Predator), and the Kingston kicks its ass all over the place. Before, I had 8x4GB sticks at 15-15-15-35 @2T. It worked sometimes, but was unstable. I mostly ran it at 2666 without problems (but I paid for 3000MHz...). The Kingston sticks are 8x8GB sticks at 15-15-15-35 @1T at 3000MHz; no problems.
Get a fast boot drive. You don't have to do this, but it won't slow down those other fast parts you have like an HDD will. M.2 is great.
You can use an HDD for data. Pro Tools can cache the timeline into RAM (disk cache may still be HD only, don't remember), so the session being on an HDD isn't a big deal. I don't know about other DAW's like Reaper or Logic. My guess is that they don't. You can still use an HDD, though. Get a fast one (7200 RPM+) and dedicate it to production, or at least partition it and use the first partition on the disk for production.
Motherboard: Go for stability, unless you need otherwise. I went with the Asus X99-E WS because it's badass and has everything I wanted; however, overclocking and hackintoshing are both difficult on this board, for some reason. Get what's right for you. Don't worry about the inbuilt audio stuff. Ignore it, unless you have a particular appellation. One reason i bought the board I did is because it has DTS live on the optical output, which I use for gaming when I'm not working.
OS: I recommend OS X Mavericks or Yosemite. That's just me. I think working with a DAW on OS X is much more enjoyable than on Windows, though you are basically your own tech support, should anything go wrong. There are other benefits, as well, but since this is highly subjective, I'll stop and let you decide.
Obviously you'll need a FW card for your interface. Do you have an interface currently? Don't get caught up in all the mobo audio triple shielded usb dec nonsense. You're a musician. Get the gear that suits your needs. Balanced cables to the monitors.
Video card is insignificant, but the Haswell-E chips do not have Integrated graphics, so you'll need something that's compatible. Easy find. Your choice. TBH, if you're not gaming, get a cheap GPU. I have a couple of HD 7750's I swap out from time to time through various machines. No PCIe power connector required, and the Sapphire one is quiet as can be. Great card, and cheap, at this point.
PSU and case are also up to you. Get what you need. @jonnyguru's excellent reviews can help with that, if you're unsure.
So, parts on your budget? I'd say Intel 5820k, Noctua NH-D15, Kingston HyperX Predator 32GB 2800 MHz (same price as the 2666 on newegg...?), maybe an Asus X99 Deluxe, if it suits you, and a Fractal Def R5. That should end up landing around your budget, give or take a few dollars and cents. IMO, If it's too much, get a cheaper mobo or case.
Hope it helps,
kyle