the reason we ask for a rough idea of what you want to spend is because prices can vary widely depending on brand and what you're looking for.
in a 12x13 satellite sized speakers are fine and will be loud enough. bookshelf speakers would also work but are a bit too much for that space. i use 75w rms satellites in a 13x14 and they are more than sufficient.
since you live in the usa, i would head down to your local best buy and check out their home audio section. they have klipsch, boston acoustics and other brands on display that you can listen to in their theater room. this would give you a good idea on what kind of "sound" you like from them. if you're lucky they may even have energy 5.1's on display (havent been in the room for a long time so not sure what they have hooked up now).
i'll give two examples below of both entry level hifi and mid level hifi 5.1 setups (depending on what you consider those levels at... )
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for example, my own setup which i personally use in a 13x14ft area is the klipsch quintet iv 5.0 satelite speaker set ($500) paired with a klipsch sw450 ($250) and pioneer vsx-30 receiver ($550) to power and connect it all. plus some speaker cable, connectors and some odds and ends ($100). in total approx $1400 for a complete 5.1 system (i did spend a bit more than i needed to on the receiver, but i wanted the features it had).
klipsch is rather pricey as are most well known hifi brands however does have good sound quality to go along with the price. i've been nothing but happy with them and would buy them again (i wish i had the money at the time for bookshelf models!) if i had the chance to.
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on more of the budget end of things, you could go with the energy take classic 5.1 set ($300) paired up with a cheaper receiver from the likes of sony or onkyo for about $200 (we picked up a sony strdh540 a few years ago for this price and its been great for our other room) and have a pretty decent setup for about $550-600 once you factor in the little extras like cable, connectors, surge protector.
for $500-600 for a set of speakers and avr you will do no better than this little gem.
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about midway between the two you'll find other sets such as 5.1 sets from boston acoustics, the klipsch hd theather 500 / 600 and other products. most need a receiver and are only paired speakers though some come paired up with a decent receiver in stores.
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you also have the option of breaking out of speaker sets and just buying speakers individually from a manufacturer. most hifi speakers are sold this way.
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what you want to avoid are home theater in a box (htib) sets. these (in the long run) generally give you nothing but grief as they often do not support multiple inputs, sometimes do not support surround sound input despite having the correct ports and offer no upgradeability. i try to get people to stay away from these unless they know full well what they are getting into beforehand.
also, i'd ignore any of the bose sets as they are generally junk unless you get into the very high end stuff.