There isn't one, you need an Intel CPU (preferably an unlocked i5 or i7 with overclock) to avoid hitting a CPU bottleneck with two 390s in Crossfire. If you absolutely must buy an AMD CPU, the FX 8350 is your best bet, though you would want a good quality motherboard and an aftermarket cooler so you can overclock. Even with an overclock on the 8350, you're still going to hit a CPU bottleneck in certain CPU heavy titles.