There is an ongoing discussion on how WiiU will handle PS4 games if PS4 is using 7 gig of RAM and Wii U has 1 available. The answer really is the 720. Developers are going to want to make games hit as many consumers as possible. XBox also has a lot of support from western developers as a primary platform. The amount of RAM used for games on 720 will significantly impact the rest of the industry.
Rumors have circulated that 720 would have 4 gig for OS and 4 gig for games. If this is the case, the extra 3 gig available for games on PS4 is largely irrelevant for multiplats as developers will want the games to run on 720 as well.
If a game can run on 4, can it run on 1? Maybe. PS2 ran the same games as XBox with half the RAM. 25% may be enough with some compromises.
In the end the market will decide what is enough RAM and processing power. If WiiU sells a lot of units, and 720 is using 4 gig then few developers will design games to push the PS4's 7 gig available. It's just like Blizzard's strategy with WoW, desigining it to be played on as many systems as possible. And speaking of PC games, that will also be relevant as most multiplats are PC as well so developers will want the game to be able to hit as many of those systems as they can too.
If 720 surprises and goes with say, 12 gig of RAM with 8 available for games, this will have a negative impact on Wii U support, and Wii U will have to be an extremely viable platform to get continued 3rd party multiplats.