The performance of having a resolution of 3840 x 1024 (3 x 19" monitors) is the same as 2560 x 1600 (1 x 30" monitor). Now having 3 x 22" monitors will definitely be more taxing.
Adding a 4th monitor doesn't really give you any performance hit because it's connected to the 2nd dvi. I've tested playing games with or without the 4th monitor and the results are the same.
I thought it'd have some impact on the performance because the CPU and GPU have to deal with this additional display area whether it is or isn't on a second head. I'm using EAX1600XT Silent and a VX724 on my old rig. After connecting a 19" monitor on the second head, there was noticeable( not very significant ) performance drop in my system.
I suspect that when the overall performance of a setup breaches a certain threshold, there will be more than enough redundancy for an extra display.
Thanks for the information. Now I won't need to worry about going with quad monitors.