Well, im no pro, but i believe the first rule of thumb is that your amp should be a lower peak wattage than your subs. So if your subs are 1000wat peak with 800watt RMS - than your going to want an amp that pushes less than 1000watt peak. This is so that you dont blow your subs out.
As far as amps go, you can get a 4 chan amp which will power everything if needed. Most of your multi chan amps push HI and LOW signals, so you can use them for all speakers. For the subs its best to get a MONO ClassD, which only pushes the low signal for your subs. Honistly, if you just purchased an aftermarket head unit, you may like the sound from the stock speakers alot better, and may not need an amp to power them further. just get an amp to power your stock sub.
So the correct setup would be a 4chan for your tweets,mids, and then run a mono amp for your sub. However iv seen many people running everything from a multi-chan amp, so i know it is possible.
your going to want to look in the stickys, i belive someone posted the specs of the stock amp and subs. take a look at the specs for your sub, and then plan your amp purchase around that, Or look at the specs for the amp (which i believe is very weak) and plan your sub purchase around that. I do know your going to be looking for a 8in sub for replacement.
Im about to install a system in my X. Avic-d2 head unit, with a kenwood class-d 1800watt mono amp with 2x 12in 1900watt comp subs. I believe that the D2 will provide enough power to the tweets without the use of another external amp.