Dude, at this point a Pentium 4 is around 3-4 years old... why buy old technology? I've got a P4 3.0 that could use a bit of a boost with all of the fun VST effects and stuff I've got going on in some projects (just experiments yes, but it still takes my computer down to a grinding halt!).
Get a core duo man
Also, firewire is a protocol: a standard to transmit data between two pieces of equipment (such as your firepod and your computer). All you need in your computer is a firewire port, preferably with a Texas Instruments chipset (which is sometimes hard to figure out)