I have a Toshiba Satellite X205-S7483 Gaming Laptop along with an external 22" LCD and I am able to play with four 800x600 windows without any major problems except when I am in Org. I had just been running 2 copies and another copy on my iMac but I just last night went ahead and went a full 5 accounts and have ran 4 pretty good. I only have 256Mb Video RAM and 3Gb system RAM.

Pretty much it is all up to what you find acceptable. If you are willing to drop the texture levels and such down to minimum you should be able to run without any real problems. I spend most of my time in Instances so that helps cut down on the FPS problem too. I get around 45FPS on all four windows when I am in an instance. When I am out in the world in most zones I get around 32FPS. In Org I get around 15FPS or even less depending on the time of day.