the 55-200 vr is awful imo. i was playing with it at work the other day and i was very unimpressed. for one, it happened to be defective - when it focused, there was a really unpleasant whine from the motor. it's not that sharp. it's not fast. the build quality is poor - ALL plastic. it's not a very nice lens. i wouldn't advise you give up the 17-55 for a 55-200VR. the 17-55 is a nice effing lens.
build quality of the sigma 30/1.4 compared to the nikon 17-55/2.8? i doubt you'd see any significant differences, but i'd put my money on the 17-55 being a little more solid. in almost any case (ALMOST) nikon/canon will make a better lens than sigma. but, personally, if i was looking at a tele zoom, i'd probably buy sigma's 70-200 over nikon's. they're both super sharp, well built, but the sigma is a lot less expensive. i personally have it in my mind to buy the 30/1.4, mainly because it's a little faster than the 24/2.8 from nikon. and again, both are sharp, well built, relatively inexpensive.. but the sigma is faster.