With the gforce 1+2+3 it was the same chip with more professional drivers. Now with the GF4 line of cards, the highend versions of the quadro is actually a different chip that is much faster than the GF line(quadro 750, 900, 980). The lowend quadro(380, 550, 580, 700?) are just like the older quadros where they are just GF4 cards with more professional drivers. I think the quadro 380(possibly the 550 also) is a gf4mx, which is not much faster than a gf2.
http://www.nvidia.com/view.asp?PAGE=quadro4xgl