Figuring out life

I am slowly trying to figure out when you cross the line of "try harder" and "there's nothing you can do about it. For example, I have people calling my house all the time asking for donations. I see the St. Jude commercial about the sick kids needing donations. I have a homelss man who begs at the corner of a road that is near my house. I see the shows on the starving kids in Africa. I want to help them all. However, John and I are selling our house to save money in case he gets laid off before he can find a new job. Should I really give away money? But I was also able to buy my kids Christmas presesnts this year and there are lots of kids who get any. I am racked with guilt with being unable to help more people without hurting myself in the process. Where is that line between being selfish and being responsible?