as you said, you have a modem and it will only support one device at a time.
get a Linksys or netgear (which I much prefer) and connect it directly to the modem and the systems to the LAN slots on the router.

we will have to teach the router some tricks to allow Comcast to use it but

have no fear

oh thank you very much,
now what do i do about this netgear box thing that allows internet access to downstairs computers so they don't have to be wireless?
and how can i switch internet activity back to the other system?

by netgear box i mean Netgear xet1001 powerline encryption utility

you can always unplug one and attach the other .... but ...

you need a third box (a new router) to get this all correct

Code:

modem--- newRouter----Netgear xet1001 powerline ...... other end---systemB
              ||
              +--- wired connection to systemA

the newRouter will provide you

• a new private lan subnet (like 192.168.x.y)
• and add DHCP service for all system attach to it
  including the Xet1001 and maybe the remote systemB

Q? does systemA need to share with systemB or conversely?

I'm jumping ahead here while I have it in mind

newRouter:
set the address 192.168.1.1
set the DHCP on
set the router ip address range 3-4 (one for the systemA, and one spare)

Netgear xet1001 which is attached to the newRouter:
ip address 192.168.1.2
(notice the above take addresses 1,3,4 and the Netgear xet1001 takes 2)
set the DHCP on
set the ip address range to 101-110

the systems connected to the newRouter will have addresses less than 1.100
while all systems connected to the Netgear xet1001 will be above 1.100

yet ALL are on the same subnet
which allows full visability and sharing everywhere