hi liam
you've got some decent kit on the post for a 'beginner'
everyone can learn something I think, even a hardened pro
Mmmmm....equipment does not the photographer make </yoda>
for lots of money you can get pro 70-200 L glass, f2.8 for churches
for less money Sigma do a nice one, f2.8 stabilised. but it's BIG
I just sold and REALLY liked my cheaper alternative, sigma 50-150 f2.8
smaller and easier to use, no OS or IS on it.
However, on the cropped 7D (I have a 40D), it becomes similar in focal length to a 70-200mm
There is also a sigma 2.8 70-200 which is moderately cheap (ish) and not stabilised but would give you some more range at f2.8 throughout and also has macro option
Remember all DSLR lenses are advertised with the focal length that you would find on a full frame sensor....even if it's an EF-S lens (cropped only)
(someone will correct me if I'm wrong quick quickly I think

)
so your 17-55 is a 27-90mm even though it cannot be used on a full frame canon camera like a 1D or 5D. odd but as far as I know... true.