Agreed I have the Tamron 17-50 too. Weight is going to go up on any decent zoom.
Outside of the scope of this thread... But I second the opinion on getting an External flash. Fought against it for too long, even bought studio strobes first.
You just can't beat having a decent lens and decent external flash with which to bounce light around. Lets you shoot at faster shutter speeds and lower IS0, and the Fstop you want not what you forced to use.
You can find a thread I started elsewhere on canon flashes. I am fond of the Sigma 530DG (around $230). There are many good choices.