you can start with a strong lefit shared hosting since you will not be using that much CPU, Bandwidth, Space initially, but soon you have to move out. The reason I say start out with shared because you may change your mind on the way of the project and it's not worth spending straight up on VPS or dedicated servers. Make sure the server provider allows streaming. Additionally, if your server provider isn't offshore which allow streaming of copyrighted music, you have to purchase license for songs to be used as radio.
Server Clusters - DB x 2 Web x 4 Dual Xeon at least, 4GB ram, SATA Raptor Drives i would say. 100mbit uplink x2 ( shared obviously )