I have a couple of things to say.
First: Those Sabertooth boards, of both revisions, work with that CPU.
Proof of rev 1:
http://rog.asus.com/forum/showthread.php?37763-ASUS-Sabertooth-990FX-Rev-1-and-AMD-FX9590
Proof of rev 2:
http://www.overclockers.com/forums/...rtooth-990FX-R2-0-Amd-FX-9590-Freeze-Unstable
Second: The VRMs are stout enough. They just need a fan pointed at them. I have a low rpm yate loon fan pointed at mine, and it keeps it cool. Granted, I have a different, lower clocked chip than you, but when I was trying to hit 5GHz, I had to lay on some crazy voltages. It worked out fine. Make sure to install the AI Suite software, and check the vrm temps. Keep it under 90c! Do this while stress-testing, obviously
Third: I don't know what you have right now Mark, but you will probably need a good cooler for this chip. It will run very hot. If you don't already have a robust cooler, just undervolt it some, and maybe drop a couple hundred megahertz. It's (probably) a better bin than the 8350, so it should work out okay speed-wise.
Fourth: Skylake overclocks really well, from what I've seen from reviews so far, and can be de-lidded for much better thermals (someone on overclock.net has already done this). If you get one, you will probably be happy with it.
EDIT: I forgot to mention the RAM. I've never heard of a board being slower with the ram maxed out. I have two 8GB sticks in mine, and it's happy with the bigger dimms. Plus, the ECC works! Asus' website says it only goes up to 32GB. Maybe so. I've never had a 16GB dimm to throw at it to find out.