Turner,
Hi,
Well, initially, you can run through two routers, but you are making it complex!
What you want, is for your router closest to the internet to provide your internal IP's by DHCP and your downstream router to be just acting as a switch. Also, for simplest operation, you would be best plugging your server into the new router and as long as your clients are on the same subnet, which they should be with only one router providing DHCP to the whole network, you should have no problems with the WHS operations.
I'm not sure of Comcast's modem/routers, but I would think they will have them modified for their own network so Comcast won't recognise third-party modems, which means you won't be able to exchange their supplied one, for your own.
Colin