Well the clock speed scales more or less linear with performance, so a 4GHz Chip will be 25% faster then the same CPU with 3,2Ghz
I've played Mario Galaxy and Metroid Prime last year with a 4Ghz Haswell i5 - and both Games had 1-2 scenes where the performance wasn't 100% - but most of the Time it was Fullspeed.
The newer Dolphin Builds which were released in the past year had some really big performance enhancements - so those 3,2 Ghz may be enough - mayabe not - you'll be save with >= 4 Ghz
I've played Mario Galaxy and Metroid Prime last year with a 4Ghz Haswell i5 - and both Games had 1-2 scenes where the performance wasn't 100% - but most of the Time it was Fullspeed.
The newer Dolphin Builds which were released in the past year had some really big performance enhancements - so those 3,2 Ghz may be enough - mayabe not - you'll be save with >= 4 Ghz