Well, "legally" means that the $29 Snow Leopard Upgrade is licensed for upgrading a Leopard-based computer ONLY. Either one that came with Leopard or one that you paid the $129 to buy Leopard for.
If you want to "Legally" upgrade your Tiger based Intel Mac (like my Mac Pro1.1 and Mac Mini), then you need to purchase the Snow Leopard Box Set which comes with iLife and iWork.
Will the $29 "Upgrade" Disk upgrade Tiger directly? These people seem to think so:
http://www.appleinsider.com/article..._leopard_disc_will_install_on_tiger_macs.html
Yes, you would save some money. But it's not technically legal. Also you'd be missing out on iLife 09 and iWork 09 which come in the Box set.
"Legal" probably is not important to you if you're asking the question in the first place, but you did...
Have Fun,
Keri
PS. "Legal" is often interpreted to mean: "Software companies will prosecute you if you are using copied or mis-licensed software in a business environment but will leave home users alone". But I wouldn't bet a huge amount of money on that.
PPS. I enjoy sending money to Companies that trust me to do the right thing. Like Apple. (no serial or activation, remember?)
So I've got all my Leopard and Snow Leopard bought and paid for on both machines.