A solid state drive is fast becoming the king of speed upgrades. You can expect a fast HD drive to write at speeds of 45Mbps and read at 60Mbps. An SSD drive should clock around 200Mbps writing and 250Mbps reading.
How does that translate into start-up speeds? We measured boot time for cold to desktop at just under 30 seconds on a new Mac mini. SSDs are still quite expensive, though. You won’t see much change out of a hundred quid for a 120GB SATA III drive.
The best machines for SSD upgrades are Mac Pros – where a spare optical drive slot accepts the SATA connection. Upgrading a MacBook or Mac mini isn’t a trivial undertaking, but the rewards are blistering.