Although I'm a Canon user, before buying my 20D I did research the Nikon stable of lenses. The 18-70 that you have gets good reports and while the 18-200 is an attractive proposition in terms of zoom range and VR all the manufacturers find it difficult to make a lens with that range with the same optical quality as a zoom with a shorter range. If you want a longer zoom I would consider looking at the 70-300 VR which has the advantage of going longer and being ~£150 cheaper. For tests of both see
www.slrgear.com and
www.photozone.de
As for the dust on sensor problem, careful lens changing should reduce this, and actually cleaning your own sensor is not that difficult (although a bit nerve wracking the first couple of times
) - I use a kit combination of SensorSweep brush and SensorSwipe/pec pads/eclipse fluid as necessary from
www.copperhillimages.com who give a comprehensive tutorial on how to go about it but also see
this thread for other options