The 990fx chips usually have better power phases and have the updated bios for the fx chips. It basically the z87 board for amd
As far as the bios goes, from what I've read the 970 chipset is mostly the same, right? The difference seems to be in how much the VRMs can handle, not what the bios can do. This is not the same as comparing Intel chipsets, as the Z chipsets are the only ones that actually allow you to fully take advantage of an unlocked chip, from a bios standpoint. It's just disabled in other chipsets, because Intel doesn't want people spending less money on motherboards and still be able to overclock, and likely spend less on a CPU since you'd be able to get acceptable performance from a budget board and a budget CPU through overclocking. (and slightly off topic, I honestly think that part of the reason the mainstream socket Intel chips are getting hotter is intentional, to reduce OC performance and protect Intel's enthusiast/extreme revenue stream)
It looks like all of the 970 motherboards have 4+1 phase designs, with the exception of Gigabyte's UD3. As far as I can tell, the only difference between Gigabyte's 970 UD3 and and the 990FX UD3 is that the 970 allows for PCI-E 2.0 x8/x8 crossfire/SLI, and the 990FX has the extra PCI-E lanes for x16/x16. In addition, the 990FX has perhaps a negligibly higher HT frequency. Spec for spec, nothing else seems different on the surface, and I'm not sure what, if anything, would cause the chips to behave differently on the two boards -- unless the VRMs are of lower quality on their 970. I can't imagine that to be the case on the same brand of motherboard with a $5 price difference, though. The VRM cooling seems the same as well.
There seems to be more of a disparity with ASRock's boards. With a price difference of about $30, the 970 and 990FX extreme3's both have the same power phase design and similar specs, with the biggest difference being that the 970 extreme3 can only do a x16/x4, so SLI/crossfiring is not really feasible.
And I'm still left wondering, if you were to use a 95W 6-core Piledriver chip like the 6300, would there would be absolutely any overclocking performance difference between 970 and 990FX boards that both support a 125w TDP CPU and use the same voltage regular circuit design with 4+1 phases.
This post was edited on 9/2 at 3:14 pm