[X-Newbies] how to seamlessly use added hard drive

[Chris Beamis]

On a dual 867MHz mirrored drive doors model, with freshly installed  
10.4.7, I have the original 57.3GB hard drive with only about 1.5GB  
free right this minute, and I just installed a new 370GB IDE drive as  
the slave.  I have disk utility poised to do something but I'm not  
sure exactly what to do.  I had originally conceived of using this  
new drive purely for backup, but with the original drive nearly full  
I must use at least part of the disk for data.   So I'm thinking now  
of trying to figure out some way to mount the new disk so that if I  
go to the command line, and type in something like

 > cd /Users

then it will go to that directory like normal, except that my shell  
prompt will actually be on the new big disk at that point.  Is this  
possible?  My idea here is that we can then continue to hog up large  
amounts of disk space for pictures and tunes without ever knowing  
that our (us users') data exists on a different disk than the system,  
and hopefully then the original disk will always have lots of free  
space just for the system and applications.  A later additional step  
will then will be to get something like about 500GB hard drive space  
and back up this whole mess to it (for the backup solution I  
originally meant for this one, plus we'll soon need to backup a  
Macbook).  I found info about hdiutil at http://tinyurl.com/7yvvw but  
got overwhelmed.  Too ignorant, need handholding. :)

Am I asking for too much?  If so, what's the most convenient way to  
store user data on the added drive, especially for the little missus  
who absolutely refuses to ever, under any circumstance, RTFM or do  
anything that involves understanding what's going on behind the scenes.

Thanks for any help,
Chris