Firewire is faster than standard USB, no doubt (regular USB -1.x- is too slow for hard disks, and DVD playback, IMHO)
In theory, USB 2.0 can get up to 480 Mbps, whereas most FW systems are limited to 400 Mbps... In practice, FW performs better and relies less on your CPU. I think that FW is overall a better solution, but USB2.0 products might become more common, and be cheaper.
Some motherboards have built in USB2.0 or FW connectors, if not, you'll have to add a card (see your manual)
The fastest hard disk interface commonly available (SATA 150) can transfer data at up to 1200 Mbps (150 MBps), so USB2.0 or FW could potentially create a bottleneck with disks that exploit these interfaces at their full potential (DVD burners are way slower than this).
Thanks for the correction! This means that USB2.0 or FW are indeed potential bottlenecks with the fastest hard disks...
Here are some numbers in megabits per second (Mbps)
USB 1.1: 12 Mbps
Firewire: 400 Mbps
USB 2.0: 480 Mbps
ATA66: 528 Mbps
Firewire 2: 800 Mbps (there are actually some even faster 'standards', for more money)
ATA100: 800 Mbps
33Mhz 32bit PCI = 1056 Mbps
ATA133: 1064 Mbps
SATA 150: 1200 Mbps
Now let's compare these with the various storage options (max throughput):
CD (1x): 1.2 Mbps
CD (52x): 62.4 Mbps
DVD (1x): 10.8 Mbps
DVD (16x): 173 Mbps
One last thing to keep in mind is that most of the current hard disks do not make full use of the current max throughput (I suggest you look at www.tomshardware.com for your favorite disks). For instance, a high end Maxtor ATA133 they tested recently had a maximum throughput of 474 Mbps (average was also significantly lower)
Edited 'cause I can't get my numbers right today (thanks Blackstar!)
I believe the company KVM makes an external housing for standard internally mounted ATA hard drives. This allows you to buy the cheaper ATA hard drives, and plug them into an IEEE-1394 (Firewire) external connection. I would go that route before investing in external firewire hard drives.
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