lol... I remember bein at bestbuy looking at a wireless and some guy was there wanting to by a Wireless G router because he believed that it would speed up his internet. I said "Your internet is probably capable of only 10 mb/s tops, that thing is capable of 54 on the wireless and 100 on the wired, your router is NOT the bottleneck". but he wouldn't believe me, even though I'm a professional! (lol)
anyways, if you are so concerned, call Comcast and ask them if they've done any work/expansions in your area lately. I'm telling you, if YOU didn't change anything THEY DID, which means you won't be able to fix it.
And yes, you would HAVE to plug one router into the modem and then the second router into that. Most modems allow only 1 IP address on the inside, which is just their way of keeping control. FORTUNATELY, they haven't added packet sniffing to modems yet, so you can plug a Layer 3 device into the modem to collate all your devices into on IP addy.
At my house, we have an old Belkin wireless (but the wifi doesn't work) but it has only 2 good ports, one up, one down. So we then plugged a 5 port switch to distribute to things like 360's and computers and my Airport, so that we have a modern wifi net as well.
In total we have:
2) PC's
1) Airport
1) Mac (on the airport)
1) iPod touch (on the airport)
1) Dell Lattitude (on the airport
1) 360
If the cable company had a way of proving that, they would charge us out the wazoo.
Hurray for Layer 3 devices!