I recently purchased a non powered speaker.
This is an image of the back:

If you can't read it, it says: 300 | PROG | 150 | RMS
                                           Prosound 10 | 8 ohms

And this is an image of it:


I want to get a power amplifier for it, and i was wondering if it matters what kind of pre-amp i buy.

errr... preamp does not equal power amp... and yes it certainly does matter what kind of preamp you buy. most mixers that have XLR inputs have preamps built into the XLR jacks... not all mixers... but most of them. My suggestion is don't worry about what kind of preamp to buy, just get a mixer that has preamps and use that - good preamps usually will cost you upwards of $1000 and though you can get some fun little $50 preamps they're really not meant for a rig like this. Get a decent mixer (yamaha or mackie), plug the mixer into a good quality power amp (crown or mackie) and you're done.

in case you care about those numbers... 300 PROG means the speaker has a theoretical peak signal capacity of 300 watts and that it can handle short bursts of this signal stregth... the 150 RMS means that the speaker has a more realistic ability to handle a constant stream of up to a 150 watt signal

for more information on RMS check out

http://en.wikipedia.org/wiki/Root_mean_square