Let's compare some prices:
- OS X 10.6 "Snow Leopard" will sell for $29 if you already have 10.5 "Leopard"
- iWork and iLife each cost $79
- The current Mac OS X 10.5 "Leopard" + iWork '09 + iLife '09 Box Set costs $169
The box set for 10.6 is probably going to cost the same (Amazon.com has it listed for that much), so if you already have iWork, you'd need to pay only $29 + $79 = $108 to get both Snow Leopard and iLife '09, far less than you'd pay for the box set--plus, you can buy iLife '09 separately (for example, now, and then buy Snow Leopard later).
We don't know when Apple will release newer versions of either iWork or iLife, but both have historically tended to be done in January at the MacWorld Expo (with the exception if iWork and iLife '08, both released during a special August '07 event during which they also introduced the aluminum iMacs). However, Apple is no longer participating in the Expo, so who knows what will happen this year.
So, here is what I would recommend:
- If you are absolutely waiting to have the latest versions of iWork and iLife (let's say version '10 will be releases some time next year), then wait to buy the Box Set to get all three products: Snow Leoprd, iWork, and iLife.
- If you are happy with your current version of iWork, just buy Snow Leopard when it comes out in October. Buy iLife whenever you feel like it: now if you don't care that '09 may become outdated next year, or later if you don't mind waiting in hopes that a new version may (or may not*) be released early next year.
Oh, and keep in mind that Amazon (or other places) often have lower prices than Apple. For example, iLife is $59.99 on Amazon right now and iWork is $53.49. (This may fluctuate.)
Hope this helps!
*I am, unfortunately, quite familiar with "waiting in hopes that a new version may (or may not) be released." Last fall when there were rumors of an aluminum MacBook that was going to be released in September, shortly after the B2S promo ended, I sold my white MacBook in hopes of buying a new aluminum one soon thereafter. I sold my MacBook in mid-August; I was, of course, kept waiting over TWO MONTHS until the end of October (with my iMac G3 as my main** computer...in 2008!). These were the most painful two months of my Apple-related life, especially since I am a student and use my computer for a lot. 
**Actually, I had a couple other computers. The most convincing alternative would have been my 2004 PC laptop (still pretty good to this day except it could use a bit more RAM)--but I liked the Mac, even the G3, so much better that I couldn't do it.
