I would have to agree with everyone on this I specialize in 1st time homebuyers, and thus have had to deal with FHA as they tightened up the standards. I do know someone that can do 600, but honestly working up to 640 is going to be ideal.
My best advice for you is to find a lender who can do "rapid rescore". Basically there are lenders who will tell you to do something to get your credit score up and then not speak with you for 3months. That is a long time, and quite frankly I have not had very good success with it.
Rapid Rescore is when a lender runs your credit profile through an automated system that pinpoints EXACATLY step by step what you need to do (will tell you the exact balance to carry on a credit card for example) in order to get your credit score up in just 30 days versus 90days.
I have seen it jump 50 points before, but even if it takes you longer than 30days the results will be more effective. That is who you want to find.
If you want a referral just contact me directly through Trulia.