To start, it sounds like you got a really uber cheap set of rear speakers. Sorry. There's different ways of saying wattage used by vendors. If you look at a sine wave, the line goes up to the top then back down, through zero then loops back up. The distance from the top of the loop to the bottom loop is called Peak-to-Peak. The very cheap manufacturers will use this number, just to make their stuff look big. So your speakers are probably 500w P-to-P. Unfortunately, you really don't use the lower half of the loop, anything under the zero, that's all negative polarity. You only use the top half, and the distance from top to zero is called Peak wattage. You'll find most vendors like Pioneer, JVC, Sony, Kenwood etc basically always use Peak power ratings, which would put your speakers realistically at 250w. Then there's RMS which is the value of usable amplitude, or in sound terms, the maximum you can go up that line before distortion happens. In your speaker, being cheaply manufactured, that'll probably be around 90w RMS.
All that said, you'll need to shop for the right amp. If you go with a cheap amp, like Pyramid or a no-name-you-ever-heard-of, you be looking at @1000w, which would be 500w per channel, same as your speakers. If you go with a known, decent brand like Pioneer or Kenwood, you'll be looking at a 500w (250x2). If you have the money for the really good stuff like PPI or Olympus, MEI or Nakamichi, Denon or even Rockford Fosgate etc, you'll want to be looking at 200w (100x2)