Hard drives typically don't draw more than 20 watts or so, but can demand a bit more with first spinning up. If you have not added hardware since new, you are probably alright. But if your current PSU is already supporting more RAM, and especially a new graphics card, it is probably time to upgrade the PSU. See my canned text below for sizing up a new one.
I don't know of any program that will allow you to power down individual drives.
I have never been happy with enclosures - perhaps it is because they have all been USB, and I have never been happy with USB. Hopefully 3.0 will fix that. So I would look for an eSATA enclosure. Hopefully your computer has an eSATA port. If not, you can buy a card, or try USB.
Use the eXtreme PSU Calculator Lite
to determine your power supply unit (PSU) requirements. Plug in all the hardware you think you might have in 2 or 3 years (extra drives, bigger or 2nd video card, more RAM, etc.). Be sure to read and heed the notes at the bottom of the page. I recommend setting Capacitor Aging to 30%, and if you participate in distributive computing projects (e.g. BOINC or Folding@Home), I recommend setting TDP to 100%. Research your video card and pay particular attention to the power supply requirements for your card listed on your video card maker's website. If not listed, check a comparable card (same graphics engine and RAM) from a different maker. The key specifications, in order of importance are:
- Current (amperage or amps) on the +12V rail,
- Total wattage.
Then look for power supply brands listed under the "Good" column of PC Mechanic's PSU Reference List
. Ensure the supplied amperage on the +12V rails of your chosen PSU meets the requirements of your video card. Don't try to save a few dollars by getting a cheap supply. Digital electronics, including CPUs, RAM, and today's advanced graphics cards, need clean, stable power. A good, well chosen supply will provide years of service and upgrade wiggle room. I strongly recommend you pick a supply with an efficiency rating equal to, or greater than 80%. Look for the 80 Plus - EnergyStar Compliant
label. And don't forget to budget for a good UPS with AVR (automatic voltage regulation).