Probably 2400mhz.My gen 1 board of the same type is stable with corsair vengeance 2400. You would probably be better running a higher fsb and slightly slower ram speed if your an overclocker mine run fine at a fsb speed of 250 and ram at 2330ish mhz this way your whole system is overclocked and ram is slightly underclocked giving you better latencies. Hope this helps.
The amount of ram depends on how many sticks your using. 16 gig is for me the best option 2 sticks running dual channel at as higher speed as possible if you can use higher rated speed than board or cpu supports by underclocking it using lower volts and latecies as possible.simple answe r is 32gig i think
I've read that the memory controller on the FX isn't that great so try to use only two sticks like 2x4/8gb's.It also isn't going to do well over 1866mhz ram because of that,you might try 2133,but that seems already high enough to go for.
With four sticks you probably would need to bring the speed down to keep it running,like with 1866->1600 for instance,maybe even to 1333mhz
If gunnerali got higher speed ram working is it up to you what to try,the official specs for the motherboard are up to 1866mhz,but that may be with four sticks.