Checked the Firewire spec -- the maximum recommended cable length is actually 4.5 m (13.5 ft), so for 30 ft you might actually need that repeater ... sorry about my previous reply.
As for using a
1394b port, according to some sources Windows XP with SPack2 forces the speed down to only 100 Mbps. However, a patch should enable you to achieve the full 800 Mbps bandwidth. Look here:
Performance of 1394 devices may decrease after you install Windows XP Service Pack 2
Seems like some amount of tweaking would be required there.

I think I'll stick with 1394a.