Get the Sunfire 280R's Service Manual.
It's linked from the system's resource page in the Sun System Handbook.
http://sunsolve.sun.com/handbook_pub/Systems/SunFire280R/SunFire280R.html
Chapter #2 will guide you through the procedures.
Glance through the rest of the documentation that's offered there, as well.
Solaris is already a multi-processor aware operating environment.
You do not need to do anything to it when you add the correct cpu component.
By that I mean that you cannot mix different cpu speed modules in the same chassis,
nor can you mix UltraSPARC-III with UltraSPARC-III+ modules at the same time.
With the right parts, it will just work immediately at the next POST.