The 580 board is several cm longer than the 575, 550, 520, CC, and CCII. It won't fit without some internal case surgery. Also, the wiring harness must be re-wired to support a different configuration (IDE and other signals are present on the edge connector of the 580 and up boards). And once undertaken, there is little point in not upgrading to PowerPC at the same time and going with a 5200, 5400, or 5500 motherboard. 5200 would be easiest as it works with the stock power supply. There is some debate as to whether a 5400 or 5500 motherboard will work to the fullest extent possible with the stock power supply.

Peace,

Drew