Upgrade the video card to a 5770 + preferably a 6870 + it will only cost like 150$. I have 2 PCs and one of them is
I7 930 24 GBs of ram ATI 6870 - Loaded 8 with no fps loss didn't try more.
AMD Phenom II X6 1100T (bought it for 250$ a year ago) 16 GBs of ram, ATI 4870 - I start to get a huge FPS loss after 3 Diablos.
To run more then 1 copy your D3 folder to somewhere else rename the folder to anything. Go to diablo III.exe create a shortcut right click properties > add -launch to the end of target