Update : Only core2duo and above processors are supported. Old graphics chips like GMA 950, X3100 are not supported anymore. So if you wish to have mountain lion on your computer, you need a good dedicated graphics card > Nvidia 8 series or > integrated intel HD 3000.
  1. Should be capable of 64bit kernel boot by default.
  2. Processor should be at least core2Duo or above. 
  3. Should have a dedicated graphics processor.

Old integrated graphics processor like Intel GMA 950 or X3100 are not supported anymore.
ATI Radeon X1600 would not be supported either.