3.5" HD in openbrick ?

Hi, just a newbie question : Is it possible to use a normal 3.5" HD with 
the openbrick ? I want to build a home router/wifi AP/samba server, I 
need at least 120 Go of disk, but big 2.5" HD are REALLY expensive 
compared with 3.5" ones ( 120 Go less than 100€), and makes the 
openbrick solution very expensive for this kind of application compared 
with a VIA CL6000 based box, for example.

Is it a space problem (not enough room in the case), unsufficient power 
supply, BIOS, or IDE bus incompatibility ?

If the problem is just the space, may i consider to put one or two disks 
on an external support (or just mounted on the cover) ? powered by the 
openbrick or an external adapter ?

Thanks