Cutting it close. EQ2 is a huge resource hog. My specs are similar to yours and I only run 3 copies and I still get hiccups. I use 2 computers at the moment.

My new plan for 5-box class setup: Guardian, Dirge, Templar, Illusionist, Warlock
If I can only manage 4 instances, then: Guardian, Troubadour, Templar, Warlock

With either of those set ups, you should be able to manage all heroic dungeons with ease. Even the instance sections you shouldnt have too much trouble if you slightly over level the dungeons.