Niel is correct. Only the retail 10.6 installer will install 10.6 on a Mac Pro that currently is running 10.4.4 or later. You can't be running 10.4.1 on a Mac Pro. You sure you have a Mac Pro, and not a PowerMac G5? Or are you just dropping the zero on the end of 10.4.10 in your signature? That zero is significant if it is there. No Mac that runs 10.4.1 can run 10.6. But about half the Macs that can run 10.4.10 can run 10.6, but they must have a built-in Intel CPU.
explains other pitfalls you may run into when upgrading to 10.6.