If you're happy with your mains, you'll likely want to find a matching center from the same speaker manufacturer and if possible, the same line of speakers. If you can provide what you're using for your towers, we may be able to help offer suggestions on this.
As for using SVS as side or side and rear surrounds, these will likely be pretty decent speakers for the money, but don't base your speaker purchasing decision on the fact you're using an SVS sub. While SVS makes exceptional subs for the money and a pretty good bookshelf offering to this point, your speakers don't have to "match" your subwoofer in any way, shape or form.
My suggestion would be that if you're pleased with the sound you're getting from your existing mains, find a matching center, then grab a pair of bookshelf speakers or bipole speakers if your movie watching is more predominant than your five channel music listening... -TD