I recently built a PC from Memory Express, this is what I got
Case:
http://www.memoryexpress.com/Products/MX30870 Great case with lots of space.
Processor:
http://www.memoryexpress.com/Products/MX31558
Motherboard:
http://www.memoryexpress.com/Products/MX36364 (Has 7.1 surround so no need for a sound card.
Video:
http://www.memoryexpress.com/Products/MX35517
Ram:
http://www.memoryexpress.com/Products/MX31394
HDD: 60 GB SSD (Wish I would of least got a 120 GB) for OS and important programs only
HDD: 1.5TB secondary drive for files.
Power Supply: 750W
Cost: $1900-$2000
As for liquid cooling, it will void any and all warranty on every part you install in the computer. Just get a nice heatsinc and fan and you will be fine. (Told to me by memory express)
With the processors, there are some that end with a K and some that do not. All that the K means is that you can overclock it, but with processor speeds these days you do not really need too, and it just makes your system less stable and run a hell of a lot hotter. 20 degrees hotter or so (on mine)
I personally do not see the need of dual video cards right now, as not many games are graphically intense enough to need 2 of them, plus the one I purchased has 2.5 GB of Video Ram.
In my opinion, I built a killer system for a mid range price. I could of gone faster, but anything over what I bought you were just paying a premium for. There is no need to go stupidly expensive as it will be outdated a week after you buy it. Keep it reasonable, there is no need to have 4 hard drives, even in Raid. 2 video cards is overkill at the moment in my opinion...
The thing I like about my motherboard is that it will kill power to the video card when you are not using it (uses less power and prolongs the life), and it also has 7.1 surround sound and USB 3.0 which my case also supports.
If you have any other questions feel free to PM me as I am in memory express almost weekly and have built a few systems myself!