[19870213]

Not a physicist, if temperature of particles is movement (I think chemical bonds will break long before the following limit), then the upper limit is just below the speed of light. To make it even hotter would require infinite amount of energy. Now what that temperature is in kelvin, I don't know.

[wiml]

Temperature is more about the kinetic (and other) energy of the particles than the velocity — you can keep adding energy indefinitely, or at least until you hit some kind of planck-scale weirdness point, even though the velocity is only asymptotically approaching c.

[parineum]

Enough energy in one place will create a black hole.