That guide has some suggestions towards brands in it? What ever you get you must amp them as you're never going to get the performance expensive speakers are capable of driven straight from a headunit.
Focal,Alpine,JL,CDT and others will work with that budget and still allow enough for a small but decent amp to drive them with.It really is a waste of money upgrading the rears,in fact this can actually make the fronts sound worse.
Chris.