Probably won't matter since the iMac still has a lame-@$$ 1.5 gbps SATA controller.
- All ALU iMacs have 3 Gbit/sec internal SATA interfaces.
- Even the 1.5 Gbit/sec SATA in older iMacs is faster than the
fastest/most modern high-capacity drives (i.e., 500 GB and up),
and
much faster than any USB or firewire external drive.
To the OP: internal drive speed is a crap-shoot. There's no telling
which make/model you'll get with a new iMac. It's a safe bet that
it'll be something reasonably fast (in the 80 MByte/sec ballpark),
but probably not the latest super-fast (> 100 Mbyte/sec) models.
Also be aware that a modern drive's "maximum R/W transfer rate"
applies only to the outer tracks. The speed drops by about 50% for
the inner tracks -- so, any given drive is about twice as fast when
nearly empty as when nearly full.
LK