99Designs may be a good fit for you, but frankly if you know a good graphical designer (or know someone who does), I’d suggest you try them first.
If you are going with 99Designs, consider my experiences:
- Tragedy of the commons: on how the 99Designs system works and misbehaves
- Logo design experience: on the kind of feedback I got, and what helped and what not.
With kind regards,