It's all relative
Some would replace with seperates, some coax, and others still would remove the oem speakers and replace with nothing (audio is recorded in stereo, so there is no real reason to have more than two channels).
Personally, I would replace with coax, as I like a bit of rear fill, but don't think that seperates are cost effective as rear speakers (as you can't really here the difference from the driver's seat to the rear deck).
Nice choice on brand by the way, Infinity makes great stuff for the money!!