for ur budget, i think you need to focus on what's important. for example, if you have a regular size room, 5.1 is usually better than 7.1. U can buy higher quality speakers.
for 15W, u can get 5W for a receiver, 10w for speakers. for the bedget, u are looking at Sony S380 Bluray, $5000 Marantz SR7005, $50000 b&w CM5, CM center, VM1 surround, CM 12inch sub. or svs 12 sub.