I run completely in EC2, my suggestion would be run 2 small instances instead of micro's. Micro's are good to burst up some CPU cycles when you need it, but don't do great at I/O, which is probably where you will have bigger issues. Even small isn't great, but they will handle things a little better if you are not CPU bound.
Only way to really know is throw some sample traffic at it and test. Test with 100, 500 and 1000 users making the appropriate number of request you feel an average visitor would. Some people have posted in the past about the amount of traffic they got from being on the front page, but I am sure your mileage will vary with the subject area.