--Let's try looking at this from another perspective. How much is your electric bill per month? On level-pay plan, mine is between $62-68 in Missouri.
--You can pretty accurately measure how much electricity your P4 is using by attaching a Kill-A-Watt to it for ~$20:
https://www.google.com/search?q=kill+a+ ... =firefox-a
--If you buy a replacement, "modern" power supply for the P4, it can also help:
--BTW, I wouldn't try attaching more than (1) HD to the RPI via USB port - at only 700MHz ARM, it's not going to have a lot of oomph in the 1st place; and disk contention over USB is not something you want to get into IRL.
--That said, I can help you out with some hdparm tweaks, and maxing out the power-saving settings in the BIOS may help as well without having to drop it all the way into L2 power saving/"standby" mode. Also, turning off the P4 when not actively in-use is probably best. You could maybe use the low-power PI to download, and then once a day (YMMV) turn on the P4 for migration and streaming...
/ just MHO
Depending on the size of download, you could consider keeping the P4 in a standby (Wake on LAN) mode, and punt the downloaded file as soon as the P4 is awake. As soon as the transfer is complete, put the P4 back to low power mode.
This is certainly what I was looking to do. But is there a way to initialize a WoLAN from the RPi? I want to fully automate the process so when the download is done it will initiate a script that wakes up the P4, pushes the file to the P4 and then, after the file pushes the P4 will standby again after 2-3 minutes. This seems to be the only solution to really limit power consumption and keep my large disk space intact...