lens for low light indoor events

I need the ability to take low light pictures indoors at family events. I use a 50D. I currently have lens for outdoors I am happy with (70-200F4, 100-400 etc), I also have 50MM1.8, and the 28-135 that came with my 50D. So my need is for indoor family events, portraits (family, not professional).

I am thinking of the following options:
1. 17-55F2.8IS
2. 50MMF1.4 (use feet for zoom), 1/3 the price of 17-55
3. Invest in a good flash unit and use the 28-135

Any thoughts are appreciated.