Socket T (LGA 775), but with a tweak in the voltage settings. And I know for sure it can be use on the 946 and 965 chipsets, I don't remember if it can even use the current high-end 975X, but if GR said so, I'm sure he's right.
And your not going to want to use a 975X anyway, I'd rather buy a new processor with it's accompanying new motherboard. Especially since the 975X is rediculously expensive for what it delivers ...