'Problem' here meaning porting 3rd party titles to all these 3 system.That leaked document about the next Xbox suggested it´d have 4GB RAM, I think the Wii U will end up with around 2GB RAM (a nice increase over the 360 and PS3) and as for the PS4, who knows.

But assuming what happens is both 'Xbox720' and PS4 having 4GB and Wii U with 2GB, is it a major problem or not?....I´m a layman with this tech talk, so what exactly would be the differences?

Err, not necesarrily, in theory they'll just be able to use a lower screen resolution, lower texture resolution etc.

In theory though.

Better than 88MB vs. 512MB. I'd say. Anyways the PS2 had 32 MB and the XB had 64 MB, so proportionally - and theoretically - it is the same difference. How did PS2 multiplats stack up to the XB versions of the same games?

The biggest issue would be GPU, I think. Looks like the Wii-U will have some 400-500 GFLOPS while Orbis / Durango are supposedly going for 1.3-1.9 TFLOPS.

Not a major problem, drawbacks like that will mean games will only look worse, it should still be able to run Orbis/durango games. If it had like 1gb or less and games were being made with first and foremost 4gb of RAM in mind then maybe that could pose a major problem, but 2gb should be alright.

It might pose a problem later on in the generation when PS4 and Xbox 3's RAM itslef will be barely enough for the developers.