I switched from a 24" + 20" to a 24"+ 22" then two 24"...
my geforce 8800 GTX had some issues from 20" to 22" and I switched from the 8800 GTX to the HD 4890 along with a second 24" monitor, and it worked very well but still I tried the two 24" with the 8800 GTX in the first place and it wasn't that great... thrust me !!

as you change the clones's windows sizes, the area to draw grow.. and with this growth, the number of pixels and polygons to be handled by the video card grow too...
you have to consider those sort of things when you change your system comp