I've got an older SATA drive (not sure if it's SATA 1 or 2). Same size plug as my new SATA 3 M/B.
Are the cables interchangeable? Any decrease in performance from using a "lower" cable?
Normally nothing has changed between the cables between SATA I,II and III. From the official SATA-IO document:
The same cables and connectors used for current SATA implementations can be used to connect SATA 6Gb/s devices. SATA-IO recommends utilizing quality components to ensure data integrity and robust operation at the fast 6Gb/s transfer rate. Cables already at the threshold of 3Gb/s operating margins may experience lower performance than expected at 6Gb/s due to an increased number of resends.
You can have a look at some benchmarks here.
Yep. They're interchangeable. All SATA is backward (and forward) compatible.