the next limit then is the "transfer rate" of your RAM - plus the overhead which is required for what you are going to do with the data.
not to speak of the graphics card. which can transport maybe 10% of what a disk offers.
i only hit the border of my HDs and SSDs when offline processing many audiochannels - in a programs which does care if a package gets lost. :/
and of course for copying between disks higher speeds are something fine.
i forgot to mention in my last post that the original drives in a quicksilver offered you only 25mb/s. it was a huge step forward to buy new IDE disks in 2006 which are almost 3 times faster! and then SATA with up to 110mb/s per HD!