The router will get the static IP unless you put it into bridge mode (where the routers firewall and NAT are turned off) in which case you can assign the IP to your server, however if you have any other PCs on the local network they can't get internet access unless they go through the SBS 2008, which then needs 2 network cards and all the associated firewall software to be secure.
Just use the routers firewall and NAT and assign a static internal IP to the server. Then use port-forwarding on the router to forward any ports like SMTP, Web, FTP etc to your SBS2008's IP address. All other PCs can use internal IP addresses and get out via the router.
