Are you sure to tower it up? I prefer the standard A1200 casing, together with a SCSI-tower for the external drives. I find it the 'prettier' solution.
Important things to upgrade:
- turbocard with extra ram;
- reasonably sized harddisk;
- cdromdrive.
Make a choice between IDE and SCSI to hook up external drives before you buy a turbocard. Some turbocard come with SCSI as standard, some have it optional, some don't have it at all.
There are many more things which you can buy for your A1200, but that depends on the amount of money you want to invest: I/O-cards, soundcards, networkcard, PCI- or Zorroboards, etc.