so according to that same web site : https://www.asus.com/Motherboards/A88XMPLUS/specifications/
4 x DIMM, Max. 64GB, DDR3 2400(O.C.)/2250(O.C.)/2200(O.C.)/2133/1866/1600/1333 MHz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture, Support AMD Memory Profile (AMP) memory
I would get DDR3-2133 but price wise will probably be cheaper to find 1866mhz NOECC Unbuffered
get 2x8GB for example as your system can run dual channel (better performance on a pair of ram than single stick)
video card wise anything will be good on this because your pcie support 1 x PCIe 3.0/2.0 x16 (x16 mode)
any video card since 2000 will run on it, what do you want to perform with you pc, office work, gaming ?
for the video card 2 things need to be considered as well
1.what monitor do you use?
2. what make and model is your power supply. (to ensure it has pcie 6+2 pins cabling to support the video cards that need it