If you are just replacing the range of the kit lens and don't want to spend alot i'd get a Tamron or Sigma - 17-50 2.8, I think they both make lenses in that range for around the 500-600 dollar mark, the Canon 17-55 IS 2.8 would be my ideal pic since it's got canon USM and is "L" quality sharp but it's also close to 1200.00 so it may be more than you want to spend. I second a vote for the sigma 10-20 (i have one and it's a great lens but not a good all purpose walk around lens, it's almost fisheye like at the wide end so it's more of a specialty lens, but I really like and it's very sharp and has great color/contrast). Personally I don't think you can beat a good prime lens (once you get used to the sharp images of a nice prime lens it's hard to pick up a zoom lens), a prime forces you to be more creative, but it is restrictive and I would only add it to your lens line up once you have the 18-300mm range covered as a bare minimum. Tamron and Sigma both make decent lenses, Sigma are far nicer in the quality of build compared to the tamron and I personally don't like the slight red color cast that most Tamron lenses seem to favor (I have a Tamron 28-75mm 2.8) neither Sigma or Tamron will focus as fast or as accurate as a good Canon USM lens especially in low light and this isn't a huge problem for most things but if action or low light photography are your type of thing i'd stick to Canon L lenses if possible since that is where you will notice the big difference.
I would not worry too much about filling a 55-70mm void, zoom with your feet and fill the void that way, do yourself a favor and pick up a Canon 28mm 1.8 or the Sigma 30mm 1.4 (my fav), a Sigma 10-20mm and replace that 70-300 with one of Canons 70-200 L series lenses, good glass makes all the difference