You've probably been reading Amazon reviews of the CS2, written by shills and noobs whose only point of comparison is their TV's built-in TV speakers.The Polk system is more expensive than the Infinity, but what I'm focusing on is the CS2 center channel. From everything I read, it's a gem, and a notable upgrade over my current center and the Infinity (and I imagine even the Klipsch). Any confirmation on that, or opinions about this specific speaker are especially appreciated.
The CS2 (and its CS10/20 brothers) are the second most complained-about center speakers on this forum, after the Pioneer Andrew Jones CP-22. I would avoid it like the plague if I were you.
If you're happy with the Infinitys (especially if the Infinity center is giving you decent voice clarity) then I'd keep them, to be honest, and just spend your $500 on a muscular sub (unless you seldom watch big budget action/thriller movies in which case never mind).
Klipsch R series should only be bought from someplace with an easy return policy if you have never heard them in person---they are infamous for ear-piercing brightness/harshness esp. at volume. The newer Klipsch RP series on the other hand is said to have mostly fixed that problem.
Also do a forum search for DCM speakers, these are the current budget champs and should fit into your $500 budget cap. Of course you've never heard of them but that's why you should look into them, as opposed to mass-marketed, overhyped junk like the Polk and AJ Pioneers. The Infinity Primus system isn't bad though, if you absolutely must have everything right now. Just keep your expectations low on the sub...a decent one starts at around $400.
Another option is the Dayton Air system, by accounts not bad at all for $250:
Or, if you insist on towers: