The Mac address is a property of the Ethernet interface, so if that is provided by a card such as a PCI or PCMCIA card, changing it for a different one will change the Mac address.
The IP address is not necessarily based on the Mac address. If you get IP assignment by DHCP (and maybe PPPoE but I'm not familiar with that) you will get a free address from a pool assigned at random. You may then get to keep the same IP address so long as the Mac address of your PC does not change but this is not guaranteed. For example my girlfriend's PC will get a different IP address if she power cycles her cable modem, even though her PC's Mac address does not change. This is a pain because my firewall is set up to allow her IP address to connect to my POP3 server and so I have to change the setting whenever she does this.
-- Jim Hatfield