About the power supply, you might get away with what you have. I've slaved a drive with less, but the power relies on the sum of the whole. Meaning, if your other peripherals are using up too much juice and you install a new HD and you boot and get a black screen- then you might need to upgrade psu. Thats the advantage of an external: it plugs into the wall (120v). Flash drives are getting popular too for mobility.
Newegg.com - USB Flash Drive, USB Flash Memory Drives, 2GB Flash Drives, 4GB Flash Drive, Flash USB
But, if you have your heart set on an internal, open your case and make sure you have another one of these (if getting sata) long enough to reach where you need it to reach. Also, there needs to be an empty sata
port on your motherboard. Just to clarify for anyone following the pic below is from the power supply and plugs into the back of the new sata HD and my previous pic of the red sata wire transmits the info from HD to MB
Where you thinking Solid State Drive for your OS?
so I can access it through the My Computer folder.
Whether you decide to go with an external HD, flash drive, or internal HD you could access through My Computer