Normal HDDs are very slow, so I decided to install two SSD and RAID them.
It was easier than I thought. It took me about 45 min to complete the operation. I used 2 OCZ agility 3 240GB RAID 0 = 480GB super fast. It took the core i7 iMac only 5 min to install Mountain Lion from a USB thumb drive.
Just do it your self.