Their biggest problem is that the cheaper 5gh/s miners got way too many orders, so they are on pre-order until 2014. Avalon makes ASIC miners. I wasn't really suggesting an ASIC miner anyway, I was just saying if you were going to spend $4000 on four 7990s for mining, you might as well get an ASIC miner chugging out 50gh/s for half the price and a fraction of the power consumption.
For a high end video rendering build like this, more RAM, cores, and threads is a must. GPU acceleration helps, but it doesn't make as big of a factor as RAM capacity, cores, or threads. If you allocate those resources correctly, you will optimize your render speeds for programs such as Cinema 4D, Vegas Pro, After Effects, Premiere Pro, Final Cut Pro, etc. If you are doing color correction on this rig as well, I'd suggest getting a 10bit panel monitor along with a 10bit capable graphics card, but that would cost quite a bit more.