need advice

here is the situation.. about november 2004, i found 2 kittens in my
backyard, very small, probably still nursing, but thier mother never
came back to feed them.. i kept an eye on them for a few hours, then
when it got dark, i put out a box and a blanket for them to sleep in,
and the next morning they were still there, so i gave them some baby
food and wound up caring for them in my backyard, giving them food and
water until they grew up a bit. now, they come and go, and i still feed
them when they are outside the door crying. they are not really feral,
they are used to me and my roommates. problem is, just last week, one
of my roommates said to me 'hey that one cat is getting fat'... well
the cat appears to be pregnant, and i dont want the cat to have kittens
here, because it would start a population boom in a very short time..
here i thought i was doing a good deed, and now it has backfired. the
block that i live on wouldnt be a good place for a bunch of cats
running around, there is a busy street right around the corner. so i
called the aspca for advice, they told me to call a local spay and
neuter hospital, who may even have waived the fee, considering the
circumstances. well, they said they cannot do it (spay) if the cat is
starting to show already, so they gave me the number of a vet that they
said is 'reasonable'.. im figuring they would have to abort the
kittens, we estimated the cat is about 5 weeks pregnant. should i take
the cat to the vet and do this to prevent unwanted kittens in the
neighborhood, or should i let the cat have the kittens and try to find
them a good home (not always easy)? im kind of stuck on this, because i
dont want the cat to lose her kittens, but the alternative is probably
worse, considering they wont have much of a life running around in the
wild out here. any advice is appreciated. thanks