If SBC isn't giving you a static IP, you can't just assign yourself a static, Internet-routable IP on the WAN/Internet side of the router; that could cause problems/conflicts. However, your SBC-assigned IP shouldn't really change that often, even if it is technically dynamic, unless you're constantly turning off the actual modem and/or the router.
If it will help though, you can turn of the LAN-side DHCP server feature on the router and just assign static IPs to the computers instead. Linksys routers usually use the (private) 192.168.1. IP range and a subnet mask of 255.255.255.0, so you just need to know the address of the router itself and your SBC DNS server IPs. Give the computers IPs of something like 192.168.1.2 and 192.168.1.3, enter the router's IP as the Gateway, enter the DNS IPs, and you should be good to go.