AMD has validated the Crosshair V and Sabertooth 990FX as for use with its new FX 9590 and 9370 CPUs, however there is some advise we'd like to add for users upgrading (or even users running their FX-8350s at ~5GHz frequencies).
1) You MUST plug in the 8pin and 4pin CPU 12V power plugs: they are there for this kind of usage! A 5GHz FX CPU will certainly draw over the 300W limit on the 8pin cable. If your PSU has good OCT it will simply cut out and your system will suffer hard-reboots. If not, it will likely melt poorly constructed PSUs with cheap wiring under extensive stress, unless your PSU is specifically fitted with heavy-gauge (lower AUG rating) wiring.
2) Do not disable OCP if you're running 24/7, it will protect your CPU. If you're trying to run extreme voltage for benchmarking you will have to turn it off, but in that case we strongly recommend watercooling (or at least chilling) the VRMs.
3) As you'd expect, when pulling close to 400W the VRMs run hot. At a minimum active cooling from direct case airflow is therefore a must (and watch those temps). Still, we strongly recommend watercooling the VRMs (EK does a good block) and do NOT loop CPU into VRMs: create two loops. When you consider your loops take into account the heat you're trying to shift: we advise looking at >1 large radiators and/or even a chiller system is maybe worth considering if you're running several high-power items together.
Given the price of CPU and expected total system cost anyway, we don't feel it's unreasonable to match such a premium setup with the right hardware. After all, this is what extreme PCs are all about