You had asked about 'watts' and such.
Most subwoofers are stand-alone units. They have a built-in amplifier and are designed to operate entirely on their own. So, your A/V receiver just needs to feed it audio, not power. Better subwoofers will have bigger built-in amplifiers and a larger/better speaker. So, you don't need to worry about your A/V receiver having power to drive the subwoofer, you just need to worry about getting the most bang for your buck from whatever subwoofer you end up getting.
For that, I suggest asking a new question in the forums about the 'best subwoofer for my money' and spell out the size of the room (length/width/height) as well as your budget. A good subwoofer can often carry over well to a newer, slightly larger space. IMO, there is no such thing as having too much subwoofer. You can always turn it down, but when you want solid bass response, it is nice to have that power available.