New pc can't multibox :(

so here we go, i'v been multiboxing on my "old pc" for a month, and i wanted to crack it up a bit, soo i bought a bran new pc, but but i can't even do 2 diablo 3, I should be able to do aleast 4 maby even 6 clients in theory

im thinking it has somthing to do with my grafic card, i'v read something about directx 11, cant work well with diablo 3, but i can't really figure out what to do about it, i dont have the greatest card but i dont have the worst soo what is a girl to do ??
my spec are:
Acer Predator G3620
Intel Core i7-3770,
16GB RAM,
2TB HD,
Geforce GTX 660 1.5GB
i would like to know about fps i'v set it to 30/30 is that right ?? or is it omg U are so wrong situation ???
also i'v read that somthing about underclocking graphic card could help, but i can't really figure out how to do it, or completely what it is ??
eny help would be appreciated
thank you in advance
sincerely Nefertiti