Speaker Upgrades (back surrounds and center channel)

[Zarathustra]

Earlier this year I very much got into home audio. My first forray into the medium came this spring/summer when I bought a PS3 $499, a Onkyo 605 $399, Fluance ES-1's $299, Fluance AV-SC $69, Fluance AV-BP2's $99 (check them out at www.fluance.com). I also ordered (but have not yet gotten) the BIC Acoustech H-100 subwoofer for a cool $200 bucks (thanks to some nice people on this site) Anyway, while I'm extremely happy with my set-up, my mind is always thinking ahead for "something better".

I've been checking around and really like the more high end polk products and was thinking of getting some quality polk bi-pole surrounds and moving my surround fluances to the back back (or should I just leave the fluances where they are and have the polks at back surrounds?). I love my ES-1's, so no upgrade is needed for them. My will also be more than enough as well.

I'm just looking for some suggestions/opinions on
1. A great center channel speaker (now that I know they're one of, if not the most important piece to a home theater set-up)
2. If I should get bookshelfs or bi-pole/di-pole surrounds for the back?
3. Any good brands or models that you think are going to fit my system well (for the surrounds or center channel).

whew, I'll let it at that for now...