You know what post... I've been in the same boat for about a year now. Searching, researching, adjusting budget ect.
This is what I have learned:
If it really will sit with your computer for the next 10 years as a computer setup. Then try to find something compact and musical (Emotive Ultra 10, DIY sealed with 300-500watt plate amp).
If it will eventually be moved to HT duty or you have plenty of space then get something with some size. The bigger the box w/ ports the more bass it will produce ie bang for the buck. 90% of the time people only turn their $1000 subwoofers to 3/10. So whats the point really?
I have a real deal car audio system with 3000+ watts going to a Audiopulse Axis 15" in a professionally built box ect... its great but if I had to go back I would get something that didn't require so much effort.
My point is... DIY is great, budget subwoofers are great but overall I suggest you get something that is built really well and does the job really well and looks sexy. The Ultra 10 linked below has good quality, size and style. You can get a second one if you ever want to make a home theater system. If you dont like it you can sell it for $200 because the resale value is high compared to the Dayton version.
Or you could look around and buy a used sub from audiogoN or any classified
Ugly and cheap =
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-635
Sexy and Pricy =
http://www.emotiva.com/ultra_sub10.shtm
