There are numerous pd/freeware/other programs around that *do* provide alt-tab, app-launch from the icon-bar, and so on - either all-in-one or separately; take your choice.
(miles further up)
If you want a really powerful RISC-OS machine, the way to go is support for multiple processors. There are already a number of cheap chips containing several (usually 4) ARM cores and suitable support; 4x600MHz would be "only" about 2GHz (maybe even a touch less) once you allowed for switching losses and the like - but the power consumption (and price!) would be much lower than for an X86, so you could always fit more than one....