my 2 cents, go with the 12' and an external display (a good CRT is cheap). Take a lot of RAM, Tiger is RAM ungry.
For the software part, you can do almost everything a PC do, expect less for gaming, but more more media stuff (video, audio, animation...). The only serious software I miss on Mac is AutoCAD and Catia.
For essential software or freeware (avaible on
www.versiontracker.com or
www.macupdate.com ):
-Desktop manager (manage many desktop, like the Linux appz)
-Drop Drawer or Dragthing (my favorite launcher)
-Menu meter (know what resource is used)
-Sharepoint (share any folder, bypass Unix limitation of shared folder)
-ThinkerTool (tweak the OS)
-EsperanceDV (making RAM disk)
-sound source (if you use more than 1 audio ouput)
-FFmpegX (video encoder)
-Graphic converter
-VLC (media reader for .avi DiVX, MPEG, ...)
-MPlayer (an another media reader)
-Clix (a database of UNIX command)
-Dockless (remove appz from appearing in the dock)
-iBackup (freeware backup utility)
-Quicksilver (appz laucher and more)
-Video codec 3iVX, DiVX, FlipforMac (the last one .wmv in quicktime)
-If you know a little Linux and Unix X11 and fink with fink commander (only if you feel ready for Unix stuff)
This is a short list of what I always have on my computer. Enjoy.