$90 is enough for 8 GB RAM.
Also, here I'd like to add some points:
That CPU has 4 modules, each containing 2 cores, which share the same resources provided by that 1 module. All in all, their native RAM support is 1333MHz. 1600MHz/Cl7 is more than enough for this CPU.
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($90.99 @ Newegg)
Now why this specific RAM:
In RAM, the rule is, the higher the MHz/CL, the better. So this's the best you can get in this MHz.
Also, this CPU heats quickly, so you'll need an af cooler. And I've heard many instances of the Corsair Vengeance's heatspreaders coming in the way of the cooler. So this RAM solves that issue.
For MoBo, I think this one will suit you well:
Motherboard: ASRock 970 EXTREME4 ATX AM3+ Motherboard ($89.48 @ OutletPC)
It got Crossfire support. And all the other standard options you need. I hope it helps