I suggest you get a Core i7 930 2.8GHz D0 instead. A good board would be the GIGABYTE GA-X58A-UD3R. If you really want to spend a large amount on a motherboard, go for the ASUS Rampage III Extreme. The HD 4870X2's stock cooling is fine, if you are using it constantly though you might want to look into watercooling for the CPU, motherboard chipset (X58 NB gets very hot) and the graphic card. As for RAM, any triple channel kit DDR3 1600MHz kit would be fine. As for a HDD, go for the SAMSUNG Spinpoint F3 1TB and if you can afford it, an SSD.
SATA3 hard drives currently run no faster than SATA 2 drives. In fact, I`m not exactly sure what the difference is since the bandwidth falls under SATA2 and they are backwards compatible for connectivity.
SATA2 provides up to 300MB per second bandwidth. The fastest desktop hard drive read speed is about 160MB per second. The cache is under 300MB per second.
I have the following setup for hard drives:
1) 300GB WD Velociraptor ($200 or more)
2) 2TB WD GREEN ($110 on sale)
FYI, there is currently more support for the NVidia cards via CUDA than ATI. Eventually ATI Stream Technology will take off but it will be a while maybe. So you may find most of your Transcoding being handled by the CPU.
The best value is an 1156 setup, such as:
1) 1156 motherboard
2) i7-860 Intel CPU
3) 4GB or 8GB DDR3 (most people need only 4GB. You MAY benefit from 8GB if you process HUGE files)
4) Windows 7 Premium x64 OEM
This is just the core system. Read comments and reviews for parts.
*Note the 1156 costs less, and uses less power than an equivalent 1366. The only reason to go with 1366 is if you will have more than a 2xHD5870 setup for video cards or if you want to spend tonnes of money on a 6-core CPU.
Heat and noise aside, the best card for Video transcoding is a GTX480.
You should also note than, although it will take a while, many programs will begin to support OpenCL which is only available on the new DX11 cards. Don`t hold your breath though, as I gave up on video card Transcoding that works well. If you look back on it, ATI has been promoting it for over 5 years for every card and ATI Stream Technology is still unavailable for the most part and definitely not as it is supposed to be.
I know the 4xxx series of ATI cards now has video transcoding although I`m not sure how well it works.
The cool thing about the upcoming OpenCL, ATI Stream is that it will (eventually) be able to transcode using the CPU and the GPU fully instead of one or the other.