Much of the N900's problem with multitasking comes up when a RAM intensive program starts getting swapped out. Normally, this isn't an issue but since accesses to the eMMC are extremely CPU intensive (and all that IO happens in the kernel) we get terrible amounts of unresponsiveness. The browser is easily the greatest culprit of this problem, and I have hit it many times.

More RAM would improve multitasking significantly, as would being able to DMA from eMMCs (which I don't know if it's possible.) This is also why messing with swappiness can improve responsiveness.