Some newer hard drives are low power 5900 RPM drives. I've started buying only LP because the power consumption is about 1/3 of a typical 7200 RPM drive they're replacing. For instance, an array with ten 2 terabyte drives now takes about 65 watts in continuous use, whereas when the array had ten 1.5 terabyte 7200 RPM drives, it took around 175 watts.
I'm not sure what kind of low power IDE drives you can find, but in the systems I have which require IDE, I've been using SATA-IDE adapters which cost around $15 USD. Then you can get, for instance, a new 500 gig low power SATA drive for $50 USD, and you'll be drawing much less power.
If you could get it to fit in the Cube, that'd be a good way to draw less power, generate less heat, and get lots of space. You will have to use something like the ATA Hi-Capacity driver to access the partition set up from 128 gigs -> the rest of the drive, but it's worth it.