Just saw this thread.... I have been doing agility with my chihuahua for over a year now and she LOVES it. Any dog that likes to jump and climb will be good at agility, and most chihuahuas certainly love to jump and climb. But of course, you'll have to wait a few months - most agility classes won't allow dogs less than 6 months old, and you really should wait until your dog is fully grown (all the growth plates fused or whatever) before your dog does a lot of jumping. Definately get her knees checked, too, that's a very common problem in chihuahuas, especially the very small ones. Just tell your vet that you're hoping to start agility, and ask him to check her to make sure her body is healthy enough to do that much physical exercise.
I think the lowest jump height in most classes (I don't know for sure, I'm sure someone will correct me) is 4", so that might be all she'll have to jump. My chi is 7 pounds, and she jumps 8" (not sure exactly how tall she is). Most obsticles are just as easy for chis as for bigger dogs, the one exception might be weave poles (although they're hard for everyone, so I don't know), because the length of your dog's body is probably shorter than the distance between the poles, so your dog will get confused very easily. Besides that, I think agility is easier for small dogs than for big dogs, they don't have to worry so much about tight turns and hitting contacts. (Which is why I don't look at this topic very much, I know how to run my own dog but I will not say that I know how to teach other people to do agility!) Good luck!