Well, the last thing I want is glitching if the drive can't keep up with the demand of streaming samples directly from it, so my limited knowledge suggests to me that I should go for as high a speed as is possible.
ANYWAY - drive now bought, installed and running.
I haven't copied anything over yet, I'm sort of thinking about an overall disk management strategy, you know, what best to use for what. It'll be a while until I can legitimately persuade "her indoors" that a brand new machine is required, so I'm going to make the best of what I've got.
It's not that bad, just 3.5 years old - disk space is the only cause for concern currently, and it now totals 320GB + 320GB + 1TB + 2TB = 3.640Gb, so now it's down to me to manage it properly.
And of course, the externals will form part of a new setup in any case, so it's not money down the drain later on.