So not only have you figured out a way to upgrade the RAM, but you've also found a way for the ARM architecture to run x86 software without emulation? You should patent that as the chipmakers themselves can't do that.

Seriously - the N900 flat out doesn't have enough RAM, there is no way to change that.

Just look at how much swap space the default OS uses... then look at NitDroid and all of the other OSes that run on the N900. Every one of them is RAM-starved. Running more than one at the same time will only compound that problem.

If you followed the link I posted above, you'd see that VMWare demoed virtualization on ARM back in 2009... but it's not available yet. If they can't get it working in a marketable state on current hardware...
You also would have found (as others have pointed out) that a new ARM CPU coming out soon will have virtualization extensions that will make this much more feasible.
But... there is no way to upgrade the CPU in the N900 - so even that is pointless to think about when you're talking about this particular device.