As with most things you get what you pay for - if you want a
really good pair of active bookshelves look at the dynaudio
BM6a for around $1k off of ebay. the mackie monitors
are also very good, i can't say i've heard the behringer, but
as a company they don't have a very good reputation
(their rep is one of putting out cheap knock-off of other
companys (mackie, etc) gear).
The new Event active monitors are supposed to be good
as well. NHT makes good stuff as well. i would stay away
from (or at least audition very carefully) alesis, foxtex,
roland, yamaha active monitors...
As other folks have said, just be sure to audition these monitors
with the same CD's, and get a good soundcard - the m-audio
audiophile series (USB or firewire) have pretty good DA's for
the price.
good luck!
-carl