First question: If you have a good video, why doe you want to convert it?
Second question: What are you going to do with resulting clip? Post it on YouTube? Present it in a cinema?
Depending on your answer, you can either have great quality and no artifacts using the QuickTime Animation Codec, or you will have to live with some quality loss if you are posting a H.264 to a service like Vimeo or YouTube.
Also note that CS5, released in the spring of 2010, is now 4 years old and uses older technology for h.264 compression.