2 things I'd note:
-Motherboard - That mobo won't fit all 4 cards natively, so you'll need at least one PCI Express extender cables, and likely two (because the top and middle cards plugged in would cover up the 2nd and 4th slots). Might want to consider getting 4x PCI Express x16 extenders, but this goes into my next point.
-No case specified. Case selection is pretty important when trying to fit 4 cards on one system, especially if you plan to multipurpose it for a gaming machine.
You might want to consider building two machines; One for gaming with two of those video cards, and another cheaper system that you can run 24x7 for the other two. I recently built two systems using the following:
Mobo: $85 - ASRock 970 Extreme -
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157280Proc: $39 - AMD Sempron 145 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103888Ram: $37 - 2x2GB DDR3 Ram -
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231277Storage: $9 - SanDisk Cruser Fit 8GB -
http://www.newegg.com/Product/Product.aspx?Item=N82E16820171587PSU: $100 - KINGWIN ABT 1050W -
http://www.newegg.com/Product/Product.aspx?Item=N82E16817121070I used that PSU for 3x 7970's, but if you do 2x 7950's could get something a lot smaller, and you could also save money on your primary system's PSU as well. I think if you break it down, you'd end up spending about the same or maybe only $100 more to split it up like this, with the benefits being that you can still have half the mining going even while playing games, and you won't have to do anything crazy to fit the video cards in cases.
P.S. That 8GB thumb drive I use for the OS on my mining rigs, they run headless (server) linux. I'm not sure if that'd work if you plan to use Windows, so something to keep in mind. Also I built my systems for BTC, so the ram amount may need adjustment as I'm unfamiliar with how much ram you should have for LTC.
P.P.S. If you're building a gaming machine, I'd highly recommend an SSD for the OS drive, and a regular disk drive for a storage disk. SSD's make a HUGE difference in loading times and performance, and games that get installed to the SSD will run much faster.