If you want to upgrade your system and like Infinity Speakers.
http://myworld.ebay.com/harmanaudio/
I suggest you try to get Speakers from the Infinity Beta Series Your tweeters will match betas. I'm pretty sure they use the same tweeter.
Betas just have higher quality drivers.
A pair of Beta 50s along with a beta 360 or your current center will make a very sick front sound stage.
Then I suggest you Get a single ES250 for your rear speakers and a pair of Beta 10s for your side surrounds.
This is the setup the speakers designer recommended.
If I weren't building my own speakers and getting married. I would probably have picked up a pair of Beta 50s. They are beasts so be forewarned.
For subs. I suggest you do the Kappa build on the DIY section or get an SVS sub.