Just saw a new WD 500gb Scorpio 2.5" go into an older Vista model Toshiba Satellite A205 model that came with a Hitachi 120gb. When the first was struck by a malware the owner inquired about boosting performance from the 120gb drive and 1gb of memory it came with as well as the need for a clean install of Windows.
The pair of 512mb SoDimms were replaced by a pair of 1gb as well as the larger faster 500gb drive. The decision for a drive that size or smaller as well as the type will depend on your budget more then anything. What's your price range for a new drive?