WTG Tina,
I loved my D40, but don't really use it anymore sinceI upgraded to the D90. I have to agree with Don's assesment of Sigma ...good glass at a better price than Nikon. Of course the purist's out there will always try and pay the extra to get Nikon. That's not to say that every Nikon lens is the best. Check out
Ken Rockwell's reviews to get some good info on lenses and even your D40. Remember, with the D40 and D60 Some of the older lenses won't auto focus. They need an internal motor in the lens to auto focus. Look for AF-S, AF-I designation for Nikon and HSM designation for Sigma. Either way double check for this before you purchase.
Have fun with your new toy.