3-4 ProResLT streams at about 10MB/s each – you should be able to pull it off on a single FW800 external drive (current model, 7200rpm). Have you tried that? It’s good for 70-75MB/s and this should be enough for 3-4 ProResLT streams.
That said, getting a 2-bay RAID0 or 4-bay RAID0/5 like with SR4 should help – even if not by much.
You are correct that while GigE offers potentially faster speeds than FW800, in reality it’s rarely the case.
Alex (DV411)