You can also replace the internal hard drive with a third party SSD drive for example a very fast OCZ Vertex 2 but since your replacing a larger spinning drive with an SSD you will loose a lot of space but gain system boot and application load speed.
There is an alternative that gives you both speed and storage space. You can replace the DVD drive with a second drive. I know this sounds crazy to some people but think when was the last time you actually put a CD/DVD disk in your laptop.
If you install a third party SSD you will need to enable Trim using this application. This will help you extend the life and reliability of your drive.
http://groths.org/enabler/updates/enabler.zip
