My setup, just for reference:

i5-2500K (overclocked to 4.3GHz on air)
8GB RAM
MSI P67A-GD65 motherboard (one button overclock)
eVGA Nvidia GTX 570
64GB Crucial C300 SSD for WoW only
120GB OCZ Vertex 2 for OS
30" LCD

I use ISboxer with all 5 windows at 2560x1600. Settings for main are "Good" and all other slaves are "low" but with "good" draw distance. I could probably make the settings higher, but I hate changing when I go into big BGs. Most of the time I get 60FPS on main, 30FPS on slaves. Exceptions are Stormwind when crowded (drops a tad) and 40 man BGs (drops to around 15 on all)