There will be a point where the clocks are too low and will cause issues, but this varies from chip to chip. Halve the frequency and see if it is noticeably cooler and still capable of some 3D, for stability checking, take it from there.
Undervolting it is also an option, but I don't recall this being a software-controlled thing for an x1950, more likely a pencil mod.
I've run my 9600GT down to 300MHz from 650, 1.10v to 0.95v. This drops temps from 42 at idle down to 35.