If budget is tight, I think you'll get better sound out of a good bookshelf and sub combo than a cheap tower. Also, the $500 sub suggestion is a good starting point. Anything less will probably not be enough for a decent sized room and won't keep up with a decent bookshelf speaker. If you're into DIY, there are very good subwoofer kits that you can buy from places like Parts Express and put it together yourself. For $600 you can get a sub that performs like a $1000 sub. You just have to assemble it yourself which is not that difficult.