hi TS. i'm in the same position as you
i'm looking to get lenses for my 450D
however currentlly what i have in mind is :
Sigma 18-200 DC OS + Sigma 30mm f/1.4
the 18-200 will be used for general walk around and outdoor , while the 30mm is for indoor low light.
hope i'll be satisfied by it and maybe it'll be usefull for you too.
anyway another thing is thta i've tried to use my friend's tamron 17-50 but found that it's not quite sharp wide open.
i feel that my IS kit lens produced sharper image 
tha'ts why i don't buy the tamron.
the canon 17-55 IS is very good though, but very expensive 
Tamron 17-50 + Sigma 18-200 OS + 430 EX - ~$1125



