It all depends on what you want to do. If you are a power junkie and believe in 1000 watts of power for mids and highs (complete overkill) then you will want to go out and by the biggest darn amplifier you can afford. If you want to be reasonable then there are a few options to take.
1. 4 channel amplifier with built in crossover.
2. 4 channel amplifier w/o crossover
3. (2) 2 channel amplifiers with built in crossovers
4. (2) 2 channel amplifiers w/o crossovers
Option 1 would probably be your best choice. You don't have to foot the additional expense of buying a crossover (if you wanted to go that route). Option 2 would require a crossover purchase (unless you just wanted to continue to use the ones you already have). Option 3 and 4 are for those who are either power hungry or want a multi amp setup. It also uses up some of those precious RCA outputs from your head unit as well (but there are ways around that too).
The next thing to decide is do you want competition level clarity, street cruising beats, or just plain annoying? That of course will determine how much you will spend on the amplifier. There are so many out there that saying one is better than the other is difficult but being from Oklahoma...I definately stick with the boys from Stillwater Labs otherwise known as Kicker. However the list is long as Alpine, PPI, Fosgate, Orion, MTX, JL, Phoenix Gold, etc etc are all great products. If you are trying to stay "brand loyal" then I don't believe you will be disappointed in the Kenwood 4-channel amplifiers. I had a competition level Acura Integra that I ran a Fosgate Punch 40 for the subs and a Punch 40 for the mids/highs that worked great (0-100 class).
Best thing to do is go and sit in some of your friends trucks/cars and listen to how they have theirs setup, what they've used find what you like. It is just stupid to pick speakers at a stereo store because they sounded good on a sound board.....the same thing goes for the amplifier.