Hard Drive ?
A Firewire drive is nothing but an IDE drive, with a bridge board to translate to Firewire, and a case and cable. Therefore, connecting a given drive by Firewire has to be slower than that same drive connected to an IDE bus directly, due to the overhead of the bus and the bridge card conversion. In my experience, Firewire 400 is considerably slower in real-world transfers than direct IDE, and for large data transfers, Firewire is -way- flakier.
The plusses for Firewire are portability and handling drives larger than 120 Gb.
If you have the choice to mount the drive internally, and speed is the primary consideration, go internal.
Note that 2001 and older G4 machines and all G3s may have a problem supporting 160 Gb and larger drives internally, requiring the use of a third party driver or installing a PCI-ATA interface card.
http://www.xlr8yourmac.com/IDE/big_ide_drive_support.html