They are an excellent value. Course the B2030P is an even better one.
Anyway you're not going to want to just get their surrounds and center. Mixing their center with your L/R Sony speakers will probably sound very odd. At the very least try out a pair of their T series bookshelf speakers (or floorstanders... if you
really want) and see if you like them better than your current Sony's.
If you do, then get their center & surrounds to match, or just use your current Sony's for surrounds if you wanna keep it cheap.