The maxupgrades are good, but a little overpriced imo. Putting 1 drive in the lower optical bay is very easy.
http://www.tenthousandpercent.com/?article=65§ion=other
Quick notes about this procedure:
1. You don't need to an "L" angle sata, a regular sata cable will do.
2. You don't need to break the regular sata either, it'll fit perfectly just slightly bent. The standard generic red sata cables work great, as they have a small head (which is most important). I had black sata cables that came with my evga motherboard, but the sata head was a "clip on" type which made the head longer, making it unusable.
3. The brackets used to mount the drive in the cage are very easily found. They are standard brackets that can be found or taken out of old PCs. OR what works best are the 5.25" to 3.5" carriers that are used to hold zip drives, floppy drives, memory card slots, etc.
4. I don't remember the length of the sata cable, but it was just the standard length. Nothing smaller than 12".
The hardest part of the installation is pretty much the prep.
Contrary to the above linked article, you should remove the cpu cover and the fan screw. (note the 2006 fan only has 1 screw)
theres a really good youtube tutorial to remove parts in the computer. I'll try and find to repost.
The only thing I could not, and advise not necessary to do is remove the front fan assembly. Shi t is impossible to remove without fear of breaking. On the 2006 you need to use brute force to yank it out, vs. the 2008 which smartly just uses an extra screw.
Anyhow the top 1 screw just needs to be removed to give the fan assembly enough room to "play" in order to fit the sata cable into place. Also note, have the dvd bays removed before doing this so you can snake the sata cable down under the fan assembly to the sata port.
Once the sata port is connected, its all downhill from there.
If you need to utilize both (2) sata ports and give you 2 extra disks. Just buy the external enclosure from OWC w/ esata adapter.
http://eshop.macsales.com/item/Other%20World%20Computing/MESATATBEK/
http://eshop.macsales.com/item/Newer%20Technology/MPQXES2/
A lot of people use this method as it is the easiest. However you still need to connect the ports though.
Also note, drives connected to the extra sata ports are not recognized in bootcamp. So no windows.