Speakers at that price point rarely get proper reviews so it's hard to make a recommendation.  Those are rather inexpensive speakers and you get what you pay for.  I see this 
set of speakers and AVR for $854 US.  That's two R610, two R41, R52 center and a Yamaha receiver.  That tells me that those are very cheap speakers.
If you want a noticeable improvement that will last, I would recommend upgrading a piece at a time with units that have been well reviewed.  That doesn't mean you can't do it on a budget. For example, Audioholics reviewed the Monolith Encore T6 and also the HECO Aurora 1000 towers which are in the $800 range.  You can upgrade your front speakers first and use your existing front bookshelfs as surround speakers.  Then save up for a good center speaker to match your fronts if you can't afford to get all 3 at the same time.
If you are set on getting four speakers for $600, then you might as well go to Best Buy or Magnolia and get whatever sounds good to you.  I can't recommend those inexpensive Klipsch and would rather see people spend a little extra for better quality.