Hi, further update. Since last playing I've upgrade the iMac from an Intel Core 2 Duo 2.0Ghz to an Intel Extreme Core 2 Duo 2.8 (model X7900 to be exact) which is the fastest, most modern CPU the iMac 7,1 can support, and I ran you game again on max settings. Flawless. No fps dip even during the horror glitches later in songs. So I feel the game is more CPU intensive than GPU.
Just thought to let you know if in case in future updates you want to put in system requirements. :)