As described in the article: you need to follow the 889713 article to determine the channel of your W2k3 installation and compare it with the channel for which you obtained the key; they probably don't match ("The product keys that are used during installation are unique for each channel and cannot be interchanged with another channel.").
The channel, the SKU, the language, and the key *have* to match, otherwise you won't be able to upgrade.
If you can't obtain a key matching the channel AND the SKU of the W2k3 base install, you need to check if options 2 or 3 under "Upgrade options" will help (which obviously requires access to a CD1 of a channel and SKU matching your R2 key); "However, you must use option 2 or option 3 if the channel, the SKU, or the product key does not match the existing Windows Server 2003 installation."
