OK, back in business.
CD is right, for FX's you pretty much need the 990 chipset for best overall experience. The Thuban Phenom cores perform better on it also. The Deneb Phenoms seem the perform better on the 890FX chipset boards. Thubans will run on a few of the black socket 890 boards without a problem, as will some of the FX's. The 890 black socket boards were actually really good high end boards. Given the choice, pending chip support, I would rather run a 890 black socket board than any of the cheap 960 or 970 chipset boards. The big noted difference I find between the 990 and 890 boards is the core control and unlocking features. They must use a different technology on the different chipsets because some of the unlockable chips I have will unlock on one chipset but not the other, and vice versa. That's why you often hear me tell people to try another board if they have a problem with their unlocking. Core control is kinda the same way as far as per core clocking is concerned.
The 790 chipset is a whole different animal. That is because there are basically two different chipsets to compare. Those being the 790X and 790FX. Difference being DDR2 or DDR3 support. The 790FX is not much different than the 890FX at all. Other than CPU support, they perform roughly the same, with a slight edge to the 890 chipset IMO. Again, core unlocking tech and core control tech between the 790 and 890 is slightly different, but unlike the 990, everything I can unlock on the 890 I can unlock on the 790 also. IMO, there is no real reason to go with a 790FX over a 890FX other than price or availability. They support the exact same products until you get into the FX's, where you should really be into a 990 board anyway.
The 790X is where it's at for the AM2 AM2+ AM3 chips. Very underrated boards. These will support AM2 all the way to Thuban without a problem. The big gaff is......DDR2 support only. Probably only a big deal to benchers, but still. Core control and unlocking is on par with the 790FX and 890FX. No issues overall. Try to find a 790X board if you're a bencher and you'll see what I mean about underrated and demand.
I'll probably add to this as I think of things.
If you have specific questions, I'll try to answer if I can.
All of this is based on my reviews and benching on many of these boards. I'm not a specific manufacturer fanboy either. I run everything.