It's hard to say what's too slow but certainly one thing to insure before paying more for your service is that you're not trying to do too many things at the same time. If for example you're using an online backup and you're doing a backup operation at the same time you're trying to watch a video I'm guessing that maybe asking too much of a low-cost DSL connection.
On the other hand it's worth verifying that you're getting the speed your paying for. To do that first find out what your download speed ought to be by looking at your contract or contacting your ISP. And then going to a site like speedtest.net and verifying that the speed you're getting is close to what you're paying for.
If it is and you're still not getting enough bandwidth then yes it's probably worth increasing your speed. With most DSL providers you'll have the option of paying them a little more money and getting sometimes a lot more speed.