Yeah, you have to look at the ohm ratings of everything... If you run the speakers in series (- on first speaker to amp, + on first speaker to - on second speaker, + on second speaker to amp), you add the ohms. Most car speakers are 4ohms, so you'd be running 8ohms per channel. The amp is probably rated at 4ohms. With twice the resistance, you'd be putting out half the power. 280W is probably peak of both channels combined. RMS (constant) power is generally about half of that, so you're looking at 70W RMS per channel at 4ohms, so 35W per channel at 8ohms. That would be 17.5W per speaker. Your amp is only putting out half of its rated power, so it will run very cool.
If you parallel the speakers (both + and both - on the speakers go to + and - on the amp), the formula is 1/A + 1/B = 1/x. A and B are the ohms of each speaker, and x is the overall resistance. 1/4 + 1/4 = 1/2, so the total load would be 2ohms. Half the resistance, so you'd be getting 140W RMS per channel, or 70W per speaker. Your amp would be putting out twice as much as rated. Most can handle 2ohms/channel, but some can't. Even if it can handle it, it will get hot. It may have problems or go into thermal shutdown (if it has it) if you don't have a fan or something on it. Been there, done that.
If the tweets and mids are a set, they're often 4ohms as a set. If you bought them separate, each one might be 4ohms, and you'll have to take that into consideration for figuring stuff out.
They had a display up at Circuit City here. It was really just to get you to spend more money, but it did make a good point. Even if you have a high-end deck, the "amplifier" in it is still just a little IC amplifying the signal. Go look at a separate 50x4 amp and tell me that a single chip in your deck can do it just as well as all the stuff in the amp.
Also, if you run front and rear speakers off the same channel, you won't have any front/rear fader control. You'll basically have 4 front speakers or 4 rear speakers, depending on where you get the signal for the amp.
