5.1 Audio Speakers, rear speakers? Options

[spuncan]

ok i've finaly learned how to use protools free and im happy with that. So I'm thinking of upgrading my stereo system to 5.1 from stereo so before I even commit I wwant to know what would be considered 'better' Surround speakers or bookshelf speakers for 2 rear speakers. I would also like some recomendation on a good brand from your veiw. Oh yeah right now I have 2 12' Cerwin Vega speakers for the front thats all.