i THINK we've got all the content except for .dlls and .exe so i wudnt back up JUST YET.... leave until hl2 works then tear yourself away from it to back up
I doubt they will be adding like hundreds of megs but who knows.
If you do a backup I assume it will just download what's missing and in a few months time there will probably be 50-60M pacthes so you'll have this problem in any case.