Looks like a solid build to me. I personally would swap out the Rosewill PSU for something from Seasonic, Corsair, Silverstone, Enermax, Cooler Master, XFX, or NZXT though. I might bump the wattage up to 850 watts just to make sure there's enough power there too.
You consider crossfire 7970s a mid grade build? LOL
anyway the 3570K is the best CPU on the market so you don't need to worry about bottlenecking your GPUs and like Vortex88 said I would swap out the PSU, I like Corsair for the power supply
There should be no reason the 7970 would not fit the r4 is a decent sized case and i would get the rosewill fortress 750 platinum everything else looks awsome
uh...what's your point? Any ATX mid tower case is going to have 7 expansion slots. Two video cards will take up 4. Full Towers will have even more slots than 7. The only cases you're not going to be able to do two videos cards in are things like micro ATX and Mini ITX cases.
Great build man! I'm in agreement on the PSU type and wattage. Corsair has some great prices right now too. Maybe add a couple of extra case fans for a solid push/pull scenario to help with the heat from the GPUs?
There are lots of decent ones out there and it's fun to add some color that way, won't cost you too much either. I would check your case specs to see what size fans will fit. Remember to go as big as possible - they'll move more air and are quieter. I added four additional fans to an NZXT and pull lots of air into the bottom and back out the top = right across the mobo. :]