First, you _may_ need an IDE header if your GCT does not have one already on the motherboard... this requires soldering (Someone posted the following link about soldering... http://www.circuittechctr.com/guides/7-0.htm). You will need to check the linux-hacker board for the exact type of header connector and a source (here is a thread http://www.linux-hacker.net/cgi-bin/UltraBoard/UltraBoard.pl?Action=ShowPost&Board=aolgw&Post=127&Idle=0&Sort=0&Order=Descend&Page=0&Session=).
Second, you need a cable... check out the following link for additional links for cables ($15-$20).
Total cost ($20-$30) not including the soldering equipment.
The power for the hard drive _could_ come from the 44 pin IDE connector (pins 41 and 42, from what the below links say.) Some 3.5" hard drives draw a lot of power and the 44 pin connector may not supply enough current... (I have another type of MoBo in which this is the case.) So probably the best bet is to check the specs on the 3.5" HD or buy one which runs on low power requirements. Usually a Laptop drive is better suited for this type of application since it is low power and the HD profile is slim.
I have not added a 3.5" drive to the GCT myself so I tried to keep the included info as generic but as helpfull as possible.
Here are some info links about IDE/ATA
ATA (I think is the same as above but you should check)