From what I understand - you have to register the MAC with NTL before it will work on the STB - however, the linksys router allows you to clone the mac address of you existing netwok card (the one on the Xover cable in your PC) and will present that to NTL as its MAC, so there should be no issues at all.....
Set the Linksys to be a DHCP server, and all your local machines to be dhcp clients, including the wireless laptop.
Once done - all should be great...
I run a linksys router with a netgear WAP and WiFi connected Win98, XP and Linux laptops (only 3 connections at a time of course, as per the UAP

) and have had no problems at all - although I have a SACM not an STB..
I would recommend checking out
http://forum.nthellworld.co.uk/showthread.php?t=6129 for a discussion on setting up a router on an STB...