in THIS case, more ram will probably help.
firewire400 is FINE. unless its just a crappy drive - and that does happen sometimes. what kind is it? im gonna guess lacie or western digital... i was using firewire400 exclusively until about 2006 and it never let me down.
how full is your firewire drive? drive performance DOES drop the more it fills up.
firewire800 and eSATA will both be better , but dose youre machine have an 800 port? and eSATA will require getting a PCI sata card.
start with the easiest first. get about a total 2 gigs of ram.