Midcow2 makes an excellent suggestion. It's better to get what you want then to rush into it. SVS and Hsu get good reviews and are well liked around here. I have an SVS and am extremely happy with it - but my model costs $1400 and isn't in your current price range. That said, there are a number of brands out there that have happy customers.
One thing that I really like about Circuit City (as well as other stores) is that you can generally buy stuff there and, if you don't like it, return it without a restocking fee. Some things have that fee, but I don't think that subwoofers do (be sure to check, though!). If you try out that Velodyne, or a Polk, or an Infinity, or whatever - and you don't like it, then no worries. You just take it back. I know a number of people that are happy with their subs that didn't cost much. A friend of mine has a Polk that I think cost about $150, and I think that it sounds good in his system (and his family room is pretty large).
Have fun shopping! For a first sub (well, second for you, I'm guessing), you certainly don't need to get the best thing out there to really think that you got something special. Everything is relative. To go from no bass, or bass from a HTIB, to something decent is a nice change that should make you and your husband happy. You can always upgrade in the future if the desire hits you and you have the funds.