Yeha, that's very old. You have two choices, rent one from Comcast (are you doing that now?), or buy your own.
When you rent, you don't get a choice of model usually, they just give you what they have that's supported and works. If the modem dies, they just give you a new one, etc. Cost is about $7 per month.
If you buy your own, you don't pay the $7 per month, but you do have to buy the modem outright somewhere, and that ranges from $30 to $90, depending on model. Personally, I would only recommend a DOCSIS 3.0 modem to give you the longest usable lifetime, but those are the more expensive models as well, usually $80-$90. I like the Motorola SB6120 (I have one here), Amazon sells it for $87. The Zoom 5341 is aout $82 over at Amazon, etc. You can look around and find slightly cheaper prices, but not by much which is why I usually stick with Amazon or Newegg. Your modem would pay for itself in about a year with saving the $7/mo rental charge. Older DOCSIS 2.0 modems are still supported by Comcast and work with the ~20Mbps speed tiers and are cheaper, like the Linksys/Cisco CM100 for around $30-40, but they will not work with the new fast tiers and they will be obsolete sooner than the DOCSIS 3.0 modems.
So you have to consider the monthy charge vs buying it outright, and also the risk that if it goes bad at some point you will have to buy or rent a replacement. Personlly, I think it's worth the risk since these devices are pretty reliable as long as you don't have local power issues which can fry them if they aren't protected adequately. Typical life on a modem is 3-5 years and you can save yourself upwards of $80-$90/yr in rental fees. And consider the fee will likely go up in the future, which will save you even more.
You do have to call and get your modem registered if you go the own route, you can't simply plug it in and away you go. Some places will allow you to do a self-registration, but that doesn't always work. Calling to register the modem usually takes about 10-15 minutes if you get a competent tech, just make sure you ahve the serial number and HFC MAC address when you call.