I said main limitatoin, not only limitation. In the average computer the hard drive is the limiting factor, speed it up and you speed the sync speed. You can, of course, get into situations where your hard drive speed is not the limiting factor. (I haven't yet)
Exactly. Reindexing on a 3Ghz machine with 1GB of RAM and a 5400 RPM drive is going to be slower than reindexing on a 2 Ghz machine with 8 GM of RAM and a fast SDD. A good CPU helps some especially on the more recent blocks (which are larger, and have more txs to validate) but someone looking for the best bang for their buck when upgrading a system should look at getting a good SSD.