The Cell has a ppc core (PPE) and 8 SPEs. The SPEs are not legacy processors and are extremely fast.
STI stands for Sony, Toshiba and IBM.
The PPC architecture has nothing to do with X86.
Using SPEs can be a significant boost to your code, but even running code on the PPE only would be several times faster than any of the A1 solutions.
Technically speaking it should be fairly straightforward to port OS4 to PS3 and a competent PS3 developer can do it.
OS4 can run as a guest OS on the system, just like linux.
You'll need SCE's support for this though.
Peripherals such as keyboard and mouse are supported by the system out of the box.
The UK price will be 425, not 500.
Finally, it is fairly easy to predict that quite a huge number of PS3s will be sold. Consoles on today's market sell more units in a few days than consumer electronics products used to sell in Amiga's heyday in years.