Arriving at optimum EC2/S3 configuration

I need some inputs on load testing to arrive at an optimum configuration for an upcoming video hosting project.

I have selected EC2/S3 for hosting, I have an idea of the load to expect - in terms of number of concurrent users, the size of files they would download, average and peak data transfer rates etc.

The problem I am facing is that I am not able to translate that to an educated guess on EC2 server farm configuration and whether a standard S3 configuration would be enough to serve my needs.

Is there a way, maybe by using exisiting load testing tools, I can test the load on the EC2 instances working in conjunction with S3 to figure out the following:

1) Number of EC2 instances needed to front the web traffic
2) Backend Database deployment configuration
3) S3 bandwidth - whether it is enough for the number of concurrent users that I have or whether I need to go to a sophisticated distribution like a CDN (cloudfront)

I would like to point out that I have take care of encoding bitrate of the videos to avoid buffering issues. At this point, I am more worried about scaling he EC2/S3 config to my user base.

Any pointers in this regard is greatly appreciated.

Thanks.