The really big problem with the 6500 is the type of RAM you are limited to, not the 603 chip. It must be 2k refresh and there are no memory sticks in 2k refresh that allow you to upgrade beyond 128mb which is too lean for OS X. Until someone comes up with a way to upgrade the memory controller to recognize 4k refresh memory OS X will always run horribly on the 6400/6500.
I saw a website from a guy in Germany who managed to get Tiger running on a 604 based Powermac that didn't have the 128mb memory limitation of the 6500 and it ran fine given enough memory. The 603 and 604 are similar enough that a 603 machine with enough memory should also work fine but given the fact that the 6500 was the pinnacle of 603 Macs with it's memory limitation we won't likely ever see a 603 running OS X acceptably.
I saw a website from a guy in Germany who managed to get Tiger running on a 604 based Powermac that didn't have the 128mb memory limitation of the 6500 and it ran fine given enough memory. The 603 and 604 are similar enough that a 603 machine with enough memory should also work fine but given the fact that the 6500 was the pinnacle of 603 Macs with it's memory limitation we won't likely ever see a 603 running OS X acceptably.