Well you've got:
AF-S 18-70 - £180
AF-S 18-135 - £250 (I think)
Af-S 18-200 VR - £400?
17-55 2.8 - £800
Sigma 50-150 2.8 - £450 - could be used with one of the above
Sigma 17-70 - not sure on this one, but peeps on here like it
Tamron 17-50 2.8 - supposed to be very good
Tamron 28-75 2.8 - again supposed to be very good.
Then you have the priimes:
Sigma 30mm 1.4
Nikon 35mm f2
Nikon 50mm 1.8 - like you already have
Nikon 50mm 1.4 - faster, better built version of 1.8 above
Nikon 85mm 1.8 - also a 1.4 version available, but its pricey.