Give your vet a call and ask him how early he will spay a cat. Some will do it so long as they weigh 2 pounds, others want to wait until the are 5-6 months old. If your is one of the later you could try ringing round, if there is a choice of vets in your area.
If you read through the kitten/pregnant cat section of the forum here, you'll learn that waiting until 5 or 6 months is fraught with danger of unintended kittens.
I think a lot of vets have perfected the techniques that allow a spay at a much younger age, and with a very small incision.
I consider 12-14 weeks to be the perfect age for a spay. Even if your vet will do it younger, I think 8 weeks is too young for a pet kitten in most cases (of course shelters do what they have to do), and kittens can go into heat at 16 weeks so it should be done before that. Call your vet and see what he/she says, just remember if they say over 16 weeks/4 months that she might go into heat and you'd have to put up with her yowling for a while. If your vet doesn't like to spay younger females you can always call around and find a vet that will do it.
It's more about weight than age. I get my litters done at 10 weeks, they are all over the required 1kg weight at that time, and then have 2 weeks to recover and have stitches removed before going to their new families.