Here's some good background from wikipedia.
1. The HDD will have a label, if it is SATA I, it will likely show SATA, if it is SATA II, it will likely show SATA II. Same cables for SATA, SATA II, and eSATA.
2. The speeds are burst speeds, so yes, you could get those speeds for short period/small file to cache before writing. It won't affect your internet download speeds. It should decrease the extract time for a WinRAR file, but I bet you won't notice it.