Use a Solid State Drive!
No I’m not kidding, Solid State Drives (SSD) improve many aspects of a PC’s performance from windows start up and responsiveness to intensive applications like SOLIDWORKS when compared with traditional spinning hard drives.
Now CATI, a US reseller of SOLIDWORKS has proved how much difference this makes for SOLIDWORKS beyond my own basic testing and subjective feedback from users. This showed improvements of 13.1-14.3% They are in the process of running a whole series of tests to show which hardware benefits SOLIDWORKS most, using a real customer dataset and testing the various aspects of a typical workflow, well worth a look at all of the tests if you have the time.
All of our standard specs at http://www.solidsolutions.co.uk/Solid-Solutions-Promotions.aspx include one SSD for operating system and programs and 1 traditional hard drive for bulk storage.
While still much more expensive per GB the price of a professional quality 256GB SSD drive has halved in the last year.
If you are looking to upgrade an existing machine I would recommend the Samsung 840 Pro Range at 128GB or above, however the biggest benefit is with upgrading the whole machine as older machines do not support the latest SATA III Standard only SATA II roughly halving the speed increase you can expect.