How Much Dare I Keep?

I've been thinking about money again lately, and the concepts of "necessity" and sacrifice. It's probably a combination of the fact that I'm currently creating my first fully-independent-of-parents budget while trying to swing downtown rent/move-in fees and replace my computer which is now (somewhat suddenly) in its last throes of death.
Last night I was describing to my dad how amazing it was to watch Les Miserables--on of my favorite stories--in London. And it was amazing. But I also remember that on the way home from the play, while walking to the tube, we passed a homeless man asking us for money. As I wrote in my journal that evening, trying to process that:
"On my way home from one of the most poignant stories of human dignity and mercy and compassion--a story which cost me a notable amount of money to see--I had 'nothing to give' the homeless man I met. ... I've been taught to love sacrificially, and I've been taught that to give money on the street is to enable drug habits and line the pockets of swindlers. It is so easy to justify inaction. I acknowledge the problems of the poor and yet always wait for a 'better opportunity' to help them than the opportunity before my eyes..."
I still don't know whether I should have given that man money that evening. Perhaps he would have used it for drugs or some other self-destructive habit. Perhaps he would have used it for food. I don't know. I do know that some people are dishonest, and prey upon people's compassion to get handouts. I also know that many, many people are hungry and suffering, and I am not.
There are other ways to help hungry people: to give them food, or food coupons, or walk with them somewhere to buy them the food or other things they need. Every time I pass an open hand or hat or battered paper cup and put nothing into it, I tell myself that: "There are other, better ways to help them." And inevitably that thought is chased by the next, that "Unfortunately, right at this moment I don't have food or coupons or time to give to them...next time." And I go on with my day, and they remain hungry.
My budget doesn't have a lot of breathing room right now, it's true. But if I know myself at all, it probably never will; because--should my income ever increase--things which now seem like unnecessary luxuries will suddenly turn into fun "splurges" and then into expected comforts, and the definition of "necessity" will subtly and nefariously raise higher and higher.
I am genuinely happy right now with what I have. I have no cause for complaint. Theoretically, if I made twice as much income as I do currently, I could continue to live as I do now and simply have that much more to give away. If only I would.
Ach, there is so much suffering and oppression and injustice in this fallen world. I pray that by the grace of God I will never be able to see it and remain content with doing nothing. (Given the mad justification-of-inaction skills I possess and less-than-ideal past experiences of wanting to save the world on my own terms, I am increasingly aware of the absolute need for it to be God at work, not myself...but those are other stories...)
-pause-
Anyway. My mind and heart are so full of these issues right now that I could go on and on, but I'll try to process in installments for the sake of anyone reading.