short note: combo drive isn't a drive that can write dvds. a combo drive just READS dvds. what you are searching for is a superdrive. the one in the powermacs is a standard IDE superdrive from pioneer. so you can just buy that one and build it into your mac. or you buy an external firewire version, like the one from lacie (www.lacie.com
i assume you're pro, so you're using dvd studio pro? but if you are working with idvd, then you would have to download a patch to get idvd to work with your superdrive.
this applies to external superdrives, but i don't know if idvd also refuses to work with internal, but upgraded superdrives? i don't know, but that's no problem anyway, cause there's this patch.