Thanks all for the feedback and info. On further review, it looks like I'd have to buy a new modem and pay $50 for activation to make the leap to DSL 50. (I don't actually mind the activation fee, I just hate having the sweaty technicians stomping around my house.)
So I might have to chew it over a bit more before making the leap.
One more question: Will DSL speeds keep increasing?
For example, I wouldn't be surprised to see a more affordable Cable 60/unlimited plan from TekSavvy in the near future (right now it's a hefty $135/month) ... and I'd be depressed if they were to roll that out after paying $200 to make the leap to DSL 50. But if DSL speeds can be increased above 50, then at least I won't feel that I blew my money on the modem and activation.