Intel will start selling this next month. 4 hot swappable hard disk on an appliance with its own processor. If you use RAID 0 and 250gb hard disks, you would have 1 terrabyte of online memory! Enough to store all your downloads plus some! Using a more robust RAID 5, you would have an error proof system, even if 1 hard disk fails.
This is being marketed for small and medium business but I got excited with its DIGITAL HOME possibilities.
Best of all, the price is $600 (without hard disks). That price break intro would surely go down over time and I expect other companies to introduce similar products. I actually saw one with only 2 hot swappable bay for $349 but I can't seem to find the site again.
Imagine not needing to transfer to DVDR's all your downloads just to be able to see them. This would be a nice addition to an ideal HTPC.
To go back to topic, you do not need to replace your power supply. Just take the additional drives outside the system.
Sure, it might still be expensive for now, but wait awhile. I would have proposed an interim solution of an external drive enclosure for your extra drives but this system uses SATA drives and I have not yet looked at existence of external hard drive systems for SATA, so I don't have any idea if its possible or if it is costly.