I'll second what VTi said about used. If you are patient and wait for Canon rebates that happen twice a year you can often get new glass less than used if you are able to combine the rebate with a B&H code. This depends on which L glass you have your eye on. Some glass is never ever going to be on rebate.
I don't know about your Sigma lenses and compatability. I very much doubt they would work on a crop camera due to the mirror. They *may* work on the 5D but if you can sell them and buy a few pieces of L glass I'd sure be inclined to head in that direction.
