It supports but there is AMD 970 chipset. There is 8x8x8 (or 8x8 - not sure). I don't know what it stands for but I read that with 8x8x8 (or 8x8 - not sure) I'll get less performance. That's mean if I will install HD7870 (crossfire) I'll get less performance?
Don't worry, technically there's no performance hit while running at X8 mode & technically there's no difference between PCIE 3.0 and PCIE 2.0 in real world perfromance, maybe you'll encounter 1% performance hit which doesn't affect real world performance.
But the cards will get bottlenecked by the AMD CPU in CPU intensive games, and you'll feel like that the HD 7870s are pretty useless, better go single card and switch to CF/SLI when you get a more powerful CPU.
This rig just needs an SSD, see if you can afford Samsung 830 128GB
Again, what's important about this whole matter is that you can't trust AMD with CF, you'll get some bottlenecks in specific games. See if you can switch to Intel they have the most powerful gaming chips.