It depends on the speakers.
You may or may not need a sub, despite what people here are telling you.
If you have small sattelites, then yes, you will need a sub.
If you have huge monitor or floorstander speakers with 8" woofers, then you could probably do without a sub.  
Depends on what you're playing out of the speakers too, and your personal preferences.
If you like the sound of explosions to rumble and shake your house, then yes, a sub would be a great addition.
Additionally, if the system is going to be a pure home theater setup for movies, then the quality of the bass isn't that important, as opposed to if it was a musical setup, thus your old HTIB subwoofer may be adequate.  
Anyways, for roughly $200, there are some decent subs out there.
$130 you can get the 12" Dayton sub at partsexpress.com
$160, there is the Velodyne VX10
And if you want something really nice, $200 will get you the 
Mirage Omni S8