I don't have comcast, but it is standard that you are using the same IP address. This is true for any DSL or cable modem/router. Having the same IP address is no issue. It's just shared with all your computers. So, you don't need another router.

Since I don't have comcast I suspect, that comcast limits how many computers can use the internet. It does this by limiting to your MAC address. You may have to spoof the MAC address on your router to be the MAC address of your computer.

What is your comcast modem connected to? A router, a hub, a switch...etc.???

What router are you using?

This is how your connection should be like:
Your comcast modem should connect directly to a router's WAN/Internet port. Then you computer should connect to the router's hub or switch.

comcast modem >> router >> your home network (include your xbox and computer)

You may have have your router configured incorrectly.