Tom's hardware has the best comparison information that I have found. From this table it would appear the current generation Intel HD integrated graphics are about equal to the lowest end discrete ATI & nVidia cards of 3 generations ago (nVidia 6600 or ATI X1400).
AndrejaKo's link does not specify which Intel integrated graphics processor the N550 uses. My recollection of the Atom N550 announcement is that it has Intel GMA 3150 graphics. That is 4 steps slower than the Intel HD graphics (nVidia FX 5200 or ATI 9200).
This all sounds disappointingly slow until you consider that unless you are playing 3D games or doing CAD or other "professional" 3D work the Intel integrated GPU is more than adequate. The Intel HD on the Core i5-6x1 chips is even certified for use by AutoCAD.