You could start with m1.small instance (ami-76f0061f, 32bit) and see if that's enough and run your database on the rds.
I highly suggest using Amazon Linux AMI, amazon has put some time in creating these Linux images, they have most of the tool that you need, plus you can easily install almost anything you need via yum. Also they are EBS backed, which has it's benefits. In in particular, making an image of EBS backed instance for load balancing is a lot simpler then S3 backed instance.
Or you can start with m1.large instance and maintain the web and the db server on the same box. Lot's of options depending on your requirements, db size, website popularity.