R730xd memory upgrade, best practice

We have a R730xd with 8*32GB Memorymodules (Dual-Rang, 2400MHz) with 2 processors. The modules are installed in slots a1 to a4 and b1 to b4. Allover memorycapacity ist about 256GB. We want to upgrade to 320GB. What is the recommended way?

Something like 4*16GB (Dual-Rang, 2400Mhz) modules put in a5,a6,b5 and b6 or is it better and possible to upgrade with 8*8GB modules? (the manual says it best performance ist given with the multipel of 4 modules per processor).

Thanks

 Stephan