For many computers, upgrading your hard drive to an SSD is the single biggest (and easiest) everyday performance improvement you can make.
SanDisk Ultra II 960GB Solid State Drive $299 (Regular $379)
SanDisk Ultra II 960GB Solid State Drive SDSSDHII-960G-G25 SanDisk Extreme PRO 960GB Solid State Drive (SSD) $389 (Regular $499)
SanDisk Extreme PRO 960GB Solid State Drive (SSD) SDSSDXPS-960G-G25
The Ultra version will be sufficient for most laptops and older desktops, whereas the extreme pro version will do best on high-end hardware. The latter is also rated to be a bit more durable.