I too was like you. I bought my camera with the 18-55mm but replaced it about 8 months later with the 24-104mm. It worked out quite well in the end since I was able to save for a L lens. The 70-300mm is a great lens and like you said you have a definite need for that lens so it is well worth getting. For everyday use though the 18-55mm isn't the best. Don't get me wrong I have seen some brilliant images with this lens but there is better. What really annoyed me about this lens is the variable aperture which is one of the reasons I choose to upgrade. As far as I have read the 17-50mm is pretty much the perfect standard lens within that price range. It has a decent focal range, constant f2.8 aperture, is quite sharp and has VC.
Heres a quick review I found. I haven't read it but The Digital Picture is a reliable review source. Don't be put off by some of the negative comments that you may find because you have to remember that this is a fairly cheap lens and that even the most expensive lenses has flaws. A quick skim through though reveals mostly very positive comments. http://www.the-digital-picture.com/Revi ... eview.aspx
I too have also considered a one lens option like the 18-270mm but they are reasonably expensive and I feel money would be better spent elsewhere and I do not want to compromise image quality. However I can definitely see why people would buy it.
Hope this helps,