I'd say "yay": buy it now.
I'm also waiting for a Snow Leopard box set. But since I'm already running Leopard, it's only iLife'09 and iWork'09 I need now.
As for the OS upgrade, I don't consider myself an "early adopter". So even when Snow Leopard is released on July and they put out a Box Set, I wouldn't upgrade the OS before the 2nd or 3rd updates come out and most bugs and problems are found & fixed. Therefore, I'd wait at least 6-8 months after Snow Leopard's initial release before I even consider putting the DVD into my mac.
So... in your case still running Tiger- and if you're not an early adopter either - that'll mean upgrading until (at least) March-June 2010, when Snow Leopard is reported stable... and that's a long time without enjoying Leopard's benefits.
cheers!