Fastest External HDD Interface?

While looking for an external HDD to hold me over 'til I build a PC in six
months or so I've been exploring external HDD interfaces. Currently, I have
a 4 year old Dell Dimension w/ XP Pro. I'm going to buy a USB2 Firewire
1394b internal card for it .I'd like to find out if 1394b is worth the
additional expense. I believe 1394b is capable of 800Mbs while 1394a is
capable of 400Mbs. A SATA II HDD (300MBs) in an external case w/ a 1394b
interface will a PC be able to make use of this high speed data transfer or
will the high data speed be wasted because the PC and XP Pro don't know how
to handle these speeds? What is the most cost effective combination for an
external HDD?

Thanks,

Mike