Sheep's got you covered. Hsu and SVS make good subs, and they will be able to outperform the Logitech sub.
That said, it's not a guarantee that you'll notice a huge improvement. It depends on what you're using it for, how big your room is, and your personal tastes. I had a PC system (Altec Lansing) with a sub that could really pound. I was highly impressed. After dropping $700 on a B&W sub (much larger, much heavier), I expected it to crush the PC sub. IMO, it didn't. However, the PC sub was in a bedroom and the B&W was in a larger living room, so kinda apples to oranges. Again, it depends on your situation. My thoughts are if you don't feel comfortable spending $500 on a sub right now...don't. You've got one already. Are there better ones? Yep. If you buy a $500 SVS or Hsu, there will still be better ones. If you buy a $2000 sub, there will still be better ones. And so on.
Now, I just upgraded from my B&W to an SVS PC13-Ultra. Without question, the SVS does indeed trounce the B&W. Crushes it. I am extremely happy. However, I did wait a couple of years until I was sure about the decision and I had the money to comfortably buy it.