Marketing is of course vital, but on it's own won't guarantee success.
I suspect in US, for example, that MS could market Natal to a decent launch, but if the games aren't there, if the people who buy it don't verbally recommend it - something that happened a lot with Wii for example - then sales will quickly die.
The same goes for Arc (or whatever it's called).
The first thing required is at least one (ideally more) titles to support the peripheral that people are going to want. Then the marketing builds on that. Imagine Wii launching with lots of marketing, the Wii mote but no Wii Sports - I doubt it would have seen everyone recommending 'you've got to try this'.
But yes, the marketing will be a vital part of this, but they needs the right games, otherwise Arc and Natal will end up as EyeToy like peripherals, and will not deliver the results both companies are clearly looking for.
Try to be reasonable... its easier than you think...
