I read a bunch of threads on this before my upgrade, and went this way:
1] buy a new drive, install along with current system drive. [fyi, i had to format it w/ Panther - Tiger woouldn't install on blank drive]
2] install tigger on new drive
3] after install is done, tigger will see your old system drive & ask if you want to copy user data, applications, etc. say yes.
In the 25 (?) years I've pummelled computers of all OS'es, this was FAR AWAY THE EASIEST upgrade I've EVER done. MS needs to take a page from this chapter. KUDOS TO APPLE.
There are cheaper ways to upgrade to Tiger, but this appears to be the easiest & most successful.