Not an expert on subwoofers so will just give my thoughts.
Some would say if you need a sub you have the wrong speakers whilst others insist it is vital to cover the lower octaves in a system.
If you want a floor shaker get a cheap one that is downward firing and has the biggest driver you can get. If you want one that will integrate nicely into a hifi set up then get a forward firing one that has a smaller, fast responding driver.
Your room size would indeed be nice to know and also you will need to determine which amplifier you are going to use as it is important it has the right connection possibilities for your subwoofer.
to my mind, to do it properly, you need a proper 2.1 amplifier, and there are not that many of those around.