Go for the fastest you can.

However, I don't think PB's have firewire 800 ports, only 400, so you will be limited to the slower speed. But if you plan to ever upgrade your computer you should go ahead and get the 800 because you could then take advantage of the faster speed.

The PB's FW800 isn't supposed to be the best implementation of the standard, but it will be faster than standard FW, but not as fast as the internal drive which is running at 133 MB/s whereas FW800 will be around 70-80 MB/s. Of course, a lot of the speed is related to what type of level the video is going to be output as. If it's home movie stuff, then FW/FW800 are plenty sufficient. If it's broadcast level, then you're out of luck, and HW RAID is the ONLY solution there - and even then, with SCSI's still, although SATA isn't far off being there and with the greater storage capacity.