I have worked with foster parents and children that have been/going though the foster system.
It's very difficult and will consume your lives, and what you and the child will get out of it will depend on a lot of unpredictable factors. For some people it has truly enriched and added meaning to their lives. For others, it has ruined their lives.
People tend to go into this with a very idealogical and rose tinted idea of 'really making a difference'. But you will have to be very realistic and accept that you wont change as much as you think you can. Some children will respond well, but for others the damage done in their early years will be too much to undo. So many people become disillusioned when they realise they cant change the world.
Ultimately, it depends on the child and their background, as well as your personal character. You will need levels of patience and tolerance normally reserved for saints.
If i was you, and i know this is not always practical, but a few months working in residential child care home setting would give you a taste of what sort of behaviours etc are displayed by very troubled children. And dont think you you already know, you dont. I have seen and heard things that you would not even dream off. Although in residential care homes many are un-foster-able and possibly worse than what you would expect as a foster parent, it would help a lot. If you can cope with that (even enjoy it) and imaging living 24/7, then it could be for you. A lot of the kids in care have been fostered, but broken the placement down and the foster parents could not cope. You may have very good well balanced kids, but what worked for them may well not work for very damaged children with very complex emotional needs and sense of reality. But obviously all kids are different.
Also, have a look at the private foster companies. The money is better (up to £30k a year per child) which will allow you to spend more time at home with them. Also try and find some other foster parents and talk to them about their experiences and also about what support they get (you will need it when things go wrong) and what training is available (sorry, but physical intervention might often be required to keep everyone safe in a crisis, and doing it wrong can get in a lot of trouble).
There is a massive shortage of foster parents. Partly because so few people want to and also because one of the outcomes from the Every Child Matters paper and legislation is that more troubled and disruptive kids are place in foster care, rather than very expensive private residential homes.
It would great it you did give it a go, im not trying to put you off, but i gave seen so many people who have walked into this unprepared. It could well be the best thing you ever do