True, they can't just us better graphics. Here's what they might do in a few years since we all know this isn't going to happen anytime soon.
1. We send in our Nintendo Wii's to Nintendo
2. Nintendo takes out the 'Hollywood' graphics card, and replaces it with a newer better one.
3.Nintendo sends it back to us
Or they might do this
1. We send in our Wii to Nintendo
2. Nintendo keeps that old Wii and sends us a new one, or we give them tons of information about our current Wii (and ourself) and just send us a new one.
There ya go. THere's how you can get COD4 on the Wii. Although in a couple years none of us will really care much about COD4, because there will be much better graphics.