They don't share the same spec, the X600XT has 350+mhz memory X600PRO has 300mhz memory and the X300 and 300SE have 200mhz memory (the SE being 64bit). Also the cores of the chips are different speeds, The X600XT is 500mhz, the X600Pro 400mhz, and both the X300s 325mhz.
There's also the mfr process difference, with the X600 being LOW cal 0.13 and the X300 non-low-k 0.11, and the difference in the quality of the HSF assembly. The mfr process difference seems to favour the X600s as the Low-k process seems to offer a nice deal more overhead.
But if you can overclock a regular X300 w/ 128bit memory to either X600 level they should perform similarly, however I doubt the memory is even close to equal quality, so that's unlikely.
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