I just did some tests to see the differences between FW800, FW400 and USB2 on our new LaCie d2 Quadra External Hard Drive:
http://www.lacie.com/products/product.htm?pid=10869
I did this test on a MacBook Pro and the drive is formatted with Mac OS Extended (Journaled) so you should not compare the results directly with an NTFS formatted drive on Windows but I'm sure the differences in speed between the interfaces will remain more or less the same.
FW800 Interface
xBench 1.3 Disk Test Score: 54.79
Sequential
Uncached Write 59.88 MB/sec [4K blocks]
Uncached Write 53.96 MB/sec [256K blocks]
Uncached Read 16.33 MB/sec [4K blocks]
Uncached Read 79.74 MB/sec [256K blocks]
Random
Uncached Write 1.41 MB/sec [4K blocks]
Uncached Write 36.00 MB/sec [256K blocks]
Uncached Read 0.74 MB/sec [4K blocks]
Uncached Read 25.40 MB/sec [256K blocks]
FW400 Interface
xBench 1.3 Disk Test Score: 47.33
Sequential
Uncached Write 39.75 MB/sec [4K blocks]
Uncached Write 35.66 MB/sec [256K blocks]
Uncached Read 13.04 MB/sec [4K blocks]
Uncached Read 36.10 MB/sec [256K blocks]
Random
Uncached Write 1.50 MB/sec [4K blocks]
Uncached Write 28.01 MB/sec [256K blocks]
Uncached Read 0.74 MB/sec [4K blocks]
Uncached Read 19.46 MB/sec [256K blocks]
USB2.0 Interface
xBench 1.3 Disk Test Score: 28.12
Sequential
Uncached Write 15.97 MB/sec [4K blocks]
Uncached Write 14.61 MB/sec [256K blocks]
Uncached Read 5.07 MB/sec [4K blocks]
Uncached Read 15.90 MB/sec [256K blocks]
Random
Uncached Write 1.55 MB/sec [4K blocks]
Uncached Write 13.87 MB/sec [256K blocks]
Uncached Read 0.67 MB/sec [4K blocks]
Uncached Read 11.63 MB/sec [256K blocks]
As you can see the USB2.0 is MUCH SLOWER than FW400. I actually expected USB2.0 to be faster, but it's not. As expected, FW800 is by far the fastest. So if you are buying a new external drive and your computer has FW800 it's well worth it to pay a bit more for a drive with a FW800 interface.