SSD RAID 0 over Firewire 800: overkill?

Firewire 800 is about the same as SATA II speed wise, I think that will be your bottleneck. I think you'd be better off getting 1 512GB SSD from a reliability standpoint. If possible, why not just replace the internal drive with a 512GB SSD?

1. Don't RAID 0 SSDs.
2. Your bottleneck is the firewire port, even for a single SSD:

http://macperformanceguide.com/Storage-Drive-SATA-vs-Fi...

It is most likely pointless to put those SSDs behind your Firewire port. It is like getting a Ferrari and then speed limiting it to 45MPH. Could you do it, sure. But why?

For offloading video in your setup get the largest disk you can that more closely matches the throughput of the port. You will get more space and realize the same performance (except perhaps for random seek times, which may come back a bit quicker).

1. Well, not over Firewire, but on a PCH or especially a real RAID controller you can go wild.
2. This is very true.