Our daughter will be 5 months on Thursday and has been strictly breastfed since birth. We just started solids on Sunday, and she LOVES it.
One of the strongest indicator's we used to determine when she was ready was her interest in our eating. I noticed her watching us about a month ago, but was trying to hold out for the 6 month mark (which is what many sources say is the best time). We could have waited, but I didn't see a reason to. She was gaining interest every time we ate.
We are only going to feed her once a day. I think she would eat more, but I believe it is suggested to not substitute their diet too quickly.
www.wholesomebabyfood.com has been an excellent resource for us. We have decided to make our own baby food, and it is so easy and cost efficient. We started our daughter on squash. Many suggest cereal, but I have not seen anything that gives a reason for that other than it's the norm.
I'm sure you'll be able to sense what is best for your daughter. I think this is a perfect example of the mother's intuition be the best indicator. Good luck!