I know someone considering a similar switch - the SCSI HDD on the 8600 is the real limiting factor here. A few other thoughts:
* I agree that the 8600 won't support FireWire target disk mode, but if you bought a FireWire PCI card and an external FireWire HDD, you could move your data and still have an external drive for your PB.
* A PCI USB card + a memory stick might also work. You'd have to price out whether this combo is cheaper or more/less useful to you in the long term than investing in a FW card & external HDD.
* A network transfer (as the previous post suggests) would also work - you'd have to share the PB's hard drive, then plug the PB and the 8600 into the same router, map out the PB's HDD on the 8600 and begin transfer (very slowly, it will be). You might be able to get away without a router if you use a crossover cable directly between the two systems, but this is just speculation.
Make sure you update th 8600 to 9.2.2 - it's not the best OS, but it will play better with whatever you decide than earlier versions. Either way you do it, you'll have to spend a few bucks to do the setup - good luck!