In terms of overclocking, generally 1 stick is better than 2. Heres an Example
1 - 512mb DIMM
Lets say the fastest speed the memory can run is 205, so your FSB could run at 205.
2 - 256mb DIMM
But when you purchase 2 256 Dimms, Lets say one Dimm can do 210, but the other 200. You will still be limited at 200, because the 2nd Dimm cannot go any higher
But with the 8RDA+, you can use 2 DIMMS and the memory will run in dual channel mode, so you get a higher bandwidth.
I believe as far as what the Motherboard can handle, I believe thats all independent of the motherboard, and the number of DIMM's do not have an effect on that.
Though the 8RDA+ is a EPoX board, and EPoX boards are known to hit 200+ FSB no problem, constantly.
Jonathan