Target Disk Mode Compatibility?, Which firewire lead is ok? Options

[Jaysee]

Hi, I've just found out about this 'target disk' trick; ie using firewire to connect two macs... when you start up computer #1 with the letter T held down, its HD icon magically appears on computer #2's desktop, hey presto! I never had much luck fiddling around with aethernet, so this sounds a lot more straight forward.
I saw the lead in question advertised on apple store UK site, hiding in a group of ads for other firewire leads of various kinds (white skinny ones)which are presumably unsuitable for the job I mentioned.
Also is this possible using a G5 to G3? Both have firewire ports but I'm unsure what to order,
I dont want yet another bit of wire around which I'll never use[SIZE=7]
thanks, J.