Just seen your reply, I would go for something about 300 watt 4 ohm output, this will leave plenty of room so you don't overdrive them,
also if your want to add another pair of speakers at a later date, you can connect them in parallel and then the load would drop to 4 ohms.
you can run a 4 ohm amp into 8 ohm speakers but a bit of power will be lost, ie the 300 watt i suggested would be about 275 watt into 8 ohms,