You can get an external USB 2.5" drive (like the Western Digital Passport) for around $110 for 160GB, $180 for 250GB. These drives are USB 2.0, and do not require an external power source. They are excellent for storing media that you don't use on a regular basis (music, videos, porn, etc).
If you're looking for internal, you can get up to 250GB or 320GB as an internal hard drive. If you're looking for speed, the fastest drive you can get is the Hitachi 7200rpm 200GB hard drive (7k200). It is the fastest mechanical hard drive you can buy for notebooks, and worth the money for its capacity and speed if you are doing things like loading a lot of apps or games.