The numbers are theoretical. USB2 at 480 Mbit/s should be faster than Firewire at 400 Mbit/s, but it's not - to the contrary. The performance one gets in practice depends a lot on the bridge chip, and in case of USB, also on the CPU architecture (USB2 drives are noticeably faster on Intel Macs - though still way slower than Firewire 400 let alone Firewire 800).
See a performance comparison
in this test of the same drive using different interfaces. I've got an enclosure with USB2, FW400 and FW800 myself and can confirm these numbers.
The fastest is of course neither USB nor Firewire, but SATA (or its external variant: eSATA). It requires buying a PCIe SATA controller though, so it's more expensive - the disks will work at the same speed as they work on the internal controller. Or even a little faster (if it's SATA II at 3 Gbit/s). So that's the best option if you want top performance. See
this thread for an example. (Note you would need a PCIe card, not PCI-X). Or check out
barefeats or
AMUG for tests of different cases, controller cards etc.
If you're on a budget, get a Firewire 800 drive with decent bridge chip (Oxford 92# comes to mind), that works excellent for backups etc.
I'm using the Firewire 800 (=IEEE 1394b) version of
this case, and put a 400 GB ATA drive in it.
Quad G5 Mac OS X (10.4.7) 4 GB ECC RAM, Raptor 150 GB, Seagate 750 GB, GeForce 7800 GT