Diablo 3 System inquire Urgent help !!!

Hi I used post a question before . but no one reply me so here I post again, hopefully some Pro-Player can reply me
so i'm currently running 2 diablo 3 window but its lagging or frozen sometimes. so my guess is maybe because my graphic card is not enough
i need someone who know more about computer setup give me some suggestion. so right now im using Nvidia Geforce GTX 550 Ti 1GB
graphic card. and my friend who is IT told me that I my graphic card is speeding up. so i'm thinking get another one, and i have SLI ready, the
thing is i'm not familiar with Radeon , the motherboard i use is ASUS P8Z68 -V LX [VGA DVI HDMI] SATA 6Gb. i was EVGA fans for many many years,
i didnt noticed when I brought my computer, so I just got my computer just couple months ago. so here is my concern and need Pro-player give me some suggestion
i'm just normal college student i cant spending too much just need need help with very common ones.
should I replace my 550Ti add another graphic card to save money? i'm very familiar with Radeon since i double check with my motherboard it only can insert Radeon models graphic card
or should i take off my current graphic card get a something like GTX680 or 690 something like that, please help this is very urgent to me or i waste a lot money >.<
sorry for the bad grammars.

You didn't mention if you reduced the graphics settings in game? might save you some money.