anamitor,
Video editing is one of the really demanding applications. 3D modeling / rendering is in that league too, but video processing can have not only the demands of rendering, but also demanding each frame being a rendering and with immense files. I showed 64GB RAM using 16GB RAM modules in my list to allow for expansion, but I'm tempted to suggest going to 128GB of RAM from the start. Since dual CPU's divide the RAM between them so 64GB total is the same as 32GB in a single CPU system.
On the subject of options, while the Quadro K5000 is my current favorite card, the older Quadro 6000 is really the video editing classic- it's that 6GB,... Of course, the new K6000 is out with 12GB and 2,880 CUDA cores (the 6000 has 448), but $5,000 is a pile of brass. The good news is that Quadro 6000's have dropped substantially, two months ago- $3,600, now $2,300 at Amazon. I saw a used sold on Ebay for $900 and I would be highly tempted. There's always a risk, but you might consider trying a pair of used Quadro 6000 at about $2500 in SLI. Imagine 12GB RAM and 925 CUDA cores on a 380-bit bandwidth. If it didn't work, 6000's will always be desirable. And these cards just run and run. I've used a Quadro FX 4800, which was $1,200 new, but purchased when two years old for $150 for about 3 years- never a hiccup. A 2006 FX 580 still runs perfectly. I don't know, really the K500 is probably the thing- 1534 CUDA cores.
These days, the Mac and Windows applications work so similarly, that I think the choice resolves to the preferred hardware. Mac seems more reliable and there's less hardware fussing, but then the user is limited to their set configurations. Windows is ugly and stupid, but you can have the hardware at just the very best way for your uses and get under the hood to fix it when it goes wrong.
Windows machines are also cheaper as is building. I priced as nearly as possible the dual E5-2687w configuration listed earlier as an HP z820 workstation and it would cost there (With Quadro K5000) a bit over $18,500. So, build yourself and save, save, save!
By the way, I meant to mention that the V2 of the E5-2687W is just becoming available. I still haven't seen the price, but the V2 is noticeably faster- instead of the V1 @ 3.1/3.8 the V2 is 3.4 / 4Ghz. Intel lists them at about $2,100.
I've only had the now old-fashioned (2009) CS4 MC Premiere- which is far more capable than I'll ever need, but not 3D capable or mobile oriented- and don't have first-hand knowledge of Grass Valley Edius. The recent V7 does have a lot of useful items, and supports 4K, 3D, and other interesting items like Thunderbolt. I'm very interested in Thunderbolt and given the potenital of 12Gb/s transfer rates, that seems just made for video editing. There are however, very, very few motherboards I've seen that have it- only one or two Haswells so far.
As for sound, that is a special interest of mine. It depends on the number of inputs and tracks, stereo or surround, whether you might be doing any remedial Foley or other recording and so on. The application is important too.
As most of my sound work is recording, I use Cakewalk Sonar- also for editing and software effects. Will you be using Audition? For high end monitoring options, you might visit Sweetwater.com as they have every level of USB, Firewire, and PCIe interface, software, and monitoring.
For monitoring and amplification I use only stereo through an M-Audio "Audiophile 192" PCI card which includes 4 I/O , duplex monitoring, MIDI and SPDIF and Protools support (About $200) through an Audio Research LS3 preamplifier > D130 power amplifier > Vandersteen 2C speakers/ Sennheiser HD 820 Pro headphones (about $5,000). That imbalance in quality with the amplification is only because I have so much audio gear about.
Again, the dividing line is whether you'll be recording, and if it's surround or stereo. For stereo, of course, headphones are best. If you can say a bit more about how you handle sound- the applications, kind of inputs, amount of effects processing, I can make more specific suggestions. I'm not knowledgeable abut Protools applications, but they still seem to be the industry standard in recording.
It's certainly possible to edit on the large scale at home. A friend of mine (using AVID) has been an assistant editor on the Spiderman series and another guy edited "Titanic" (also in AVID) in a couple of back bedrooms knocked together.
Cheers,
BambiBoom