Why don't you keep the 18-55 and get the 70-300?
From what I've heard, the 18-55 is far from 'worthless'. In fact, on his website Ken Rockwell lists it in his Top 10 Nikon Lenses, describing it as 'sharp, sharp, sharp'.
This is what he said:
'Even as a cheap, plastic lens, the 18-55mm is...