Nikon does make digital SLRs. Currently their cheapest current model is the
Nikon D50. It is going to be over $400. With a lens, you can currently get it in the $600s. They may have something for the holiday season, so wait a few weeks to see if there are any announcements.
Other ones to look at: Canon
Digital Rebel XT (they have a newer one out, the
XTi, but prices may be good on the XT),
Pentax K100D or K110D (only difference is that
K100D has their "Shake Reduction" system and the K110D does not).
If you think that you'll want to drop more money on a more advanced digital SLR body later, then a major part of your decision should be the entire product line including lenses. For example, if you get a Canon now, you can always reuse your "kit" lens when you upgrade bodies.