Holy @#$%, that is one serious set of subs you got.
Problem is, they are probably burying the rest of your system. A separate amp is going to be a must - if I'm reading the specs right, your current one is a single channel. Look for either two good 2-channel amps, or a single 4-channel. Probably better sound if you add 2 more amps, and easier to install that way imo. And I'd make that a higher priority than replacing the speakers. Unless they're absolute dog-squeeze, which they may be, running the factory speakers off a good amp will sound better than upgraded speakers using the head-unit.
In terms of speakers, this is one area where you don't necessarily want to just go listen to some at the store and pick ones that sound good. How they sound in the demo room is nothing like they'll sound in your car. Buy a quality speaker rated for a good amount of power. And ideally you want an amp that will slightly over-drive them. I.e. feed 125 watts to a 100 watt speaker. If your car will allow it, you'll get better "imaging" if you can get a component system (tweeters mounted higher on the door). Something like this
I don't personally pay much attention to rear speakers. In theory, they're only there for "rear-fill", you shouldn't actually hear
them. Buy a decent set of speakers and I'm sure you'll be happy.
What's you plan for installation? Two concerns: running that much power makes your system more susceptible to engine noise, and you want to make sure you get enough power to your amp(s). Running new good-quality speaker wire to all speakers is a must, don't use the factory wiring harness. You'll need to make sure you get a strand of 10 or 8 gauge wire to each amp, maybe thicker than that even. And I would run a ground back to the engine compartment.