I have a 28" monitor, and I can play two instances on one box @ 1920x1200 provided there is no antialiasing. When I open three instances, though, framerates begin to slide into the toilet. My system specs are as follows:

Geforce 8800GT 512
Intel Xeon 3120 Quad Core 2.0 @ 3.2 Ghz (Orthos and Prime stable for 24 hours)
Auzentech X-Meridian
4 GB 1066 DDR
Raid 0 320 GB 7200 HD
Vista x64

I try to keep the 1 instance at 1920x1200, and then scale the other two down. Unfortunately, I have to put them at 1280x720. Setting each WoW process for a different CPU core affinity, it is more fluid, but since I was interested in a new graphics card anyway, I'm wondering which graphics card would be better so that I could run all my instances at higher resolutions - one with 1GB of RAM or a dual GPU solution. I am not very impressed with the issues that crop up under SLI/XFire modes, and my motherboard doesn't have 2 PCI-Express slots. If I go with a dual GPU solution, I would have to buy something like the 3870 X2. The 9800 GX2 might or might not be good, but I don't know how much of an improvement it would be considering that it costs way more than I want to spend.

Suggestions from other tri-boxers would be greatly appreciated.

EDIT: For clarification, I didn't include my monitor. It is a 1920x1200 28". I don't have room for a second one, so I have been alt+tabbing. It works surprisingly well, and I was able to do the entire Scarlet Monastery with a group of 38's - Prot Paladin, BM Hunter, Enhance Shaman. I leveled the Paladin and Hunter together from this box alt+tabbing and using AutoHotKey. I just bought Keyclone since I am beginning to need more hotkeys with this team. With just two 1920x1200 boxes, framerates are great, and I can even use 2X multisampling AA on one window. With 3 windows, though ... AA goes, and the other two have to be reduced in resolution to very basic.