I don't know about Warcraft or Diablo, but it's easy to set a static IP for you Mac. In Network Preferences, click on the TCP/IP tab, and select "Manually" in the Configure box. You'll have to enter the IP, the subnet (which should default to 255.255.255.0 and that's ok), the "router" address (some places call this entry "gateway" but it really is the LAN address of your router) and your DNS server addresses. Click on "Apply Now" and you're done.
However, you should note that Linksys DHCP servers tend to assign the same address to a computer all the time. This is useful particularly when you only have a couple of computers connecting to the router. If it's just ONE computer, you should ALWAYS get the lowest address in the range your router has selected-by default they go from 192.168.1.100 to 192.168.1.149.