"Over matching"
First, amp output is rated at 8 ohms- observe that as much as possible when buying speakers. Lower inpedence (say, 4 or even 2 ohms) will lead to overheating your amp- not good. If you know you will always be driving only two speakers, get 8 ohmers. If you know you will be chaining speakers (two hooked up to amp, one more hooked up to each speaker, for a total of 4 speakers) get 16 ohmers- total load will be 8 ohms. You can probably do that with 8 ohm speakers, too, for a total load of 4 ohms, but most amps are not happy with a 2-ohm load.
Keep your speaker power handling rating a bit BELOW your amp's output rating, but not by much. Over powering the speakers, while not advised, is not as bad for them as driving them with a seriously clipped signal. A 100-watt amp straining to drive 500 watt speakers does more damage to them than an amp driving them with 600 watts. Overdriving can burn out coils, but clipping causes them to just come all apart. Sounds like crap, too.
For a relatively low-power rating (280 watts can get lost pretty quick in a venue) I would go with high-efficiency speakers.
For your amp, I would try speakers with about a 300 watt/ 8 ohm rating. If your pre-amp device (keyboard or mixing board) has a sub-woofer out, you might want to get a self-powered sub of about 500 watts (guessing, here) which will take care of the lows and allow your QSC to deal with mids and highs. Using that self-powered sub would also allow you to go with much smaller speakers- 2-ways with 10's or even 8's might carry all you need.
If you are coming directly out of the keyboard to the amp, a small mixing board might come in handy- it will allow you to hook up a second or third keyboard, and a CD player to keep the music going when you take a break, and allow you better control of what goes to the sub (if it has a sub-woofer out).
There may be others who can give you better advise, but this is a good start.