Nina,
You can also modify the global resource (under the host menu), which would be for all portals. You can also modify the portal resource (under the admin menu), which would be only from the single portal. If you also look into the {portal root}\App_GlobalResources, you will find all the overrides for the portal. For example, in my portal I have GlobalResources.Portal-0.resx, GlobalResources.Portal-1.resx and GlobalResources.resx. The first two is for the specific portal and the last one is the global resource. Although I haven't dug into code, but it is my belief that this is how it works. How else would you support multiple languages in the same instance?
Regards,
Phebous...
www.ricecheesesoftware.com