There is some confusion about whether this violates the license agreement however, so you might want to call Apple and get an "o.k." first.
The thing is, if you're going from Tiger, it may be wise to format your hard drive and do a clean install (by booting into the Snow Leopard disk) (although I suppose it couldn't hurt to try a regular upgrade first). And if you do erase your hard drive, you will lose your documents/apps unless you backed them up, and since iLife was pre-installed on your Mac (without providing you with a disk or install files) the only way to make sure you don't lose iLife is to buy iLife separately or along with Snow Leopard and iWork with the Box Set.
Also make sure to check out http://support.apple.com/kb/HT2980 -- If you do an erase and install, you would want to first make backups of your Firefox profile (if you use Firefox), the Mail library folder/preferences to make sure you don't lose your emails, etc.
I just upgraded a first generation Intel Macbook Pro running 10.4.11 to 10.6.3 using the $29 Snow Leopard upgrade disk and haven't had any problems yet. I didn't really want to upgrade but for some bizarre reason the iPhone 4 forces an update (even though my old iPhone 3 GS running iOS 4 and iTunes 9.2.1 had no problem with Tiger).
You can use a retail Snow Leopard disc to install SL on your system, even if you're running Tiger, so long as you have hardware capable of running SL. You will not have significant problems going from Tiger to SL.
However, it should be noted that it is a really good idea to have a +complete+ backup of your current system before you upgrade. Just in case something happens. It's most likely that nothing will happen, but why take the chance?
You don't have to reinstall anything, as you don't have to do an erase install. You should back up before installing, just in case, but the normal install is an in-place install which won't do anything to any existing applications. Depending on how old your apps are, you _might_ have to install Rosetta, which allows older apps to run, but that's an optional install on the install disc, and even if you forget to install it, the first time you need it, it will be installed either by being automatically downloaded over the Internet or by asking for your install disc. Not a big deal.
If you do decide to do an erase install, one of the options available on doing that install is do a setup using your backup. The install disc will ask you to connect your backup disk to the Mac and will read your backup and move over all your settings, files, and (unlike Windows) apps. When it's done your Mac will be exactly as it was before you did the SL upgrade except that it'll be running SL instead of Tiger.
Office 2004 and X will work in SL. Office 98 might have a problem, though.
I just bought OS X Snow Leopard 10.6.3 at the Apple store today for $29 and updated my iMac (Intel-based) 10.4.11 without a hitch. The salesperson at the store was doubtful it would work. Ha! So much better than having to shell out $170!
If by 'clean install' you mean 'erase the drive and install and then restore your apps and data', then you will be doing a lot of work for very little, if any, gain. OS X is _not_ Windows. It will work perfectly adequately with just an upgrade install. The machine I'm typing on is an older Intel iMac, which came out of the box with 10.4.something. I upgraded it to 10.5 using the plain ordinary upgrade install. I later upgraded it to 10.6 with the standard upgrade install. Next to it is a machine which someone else, who had considerable experience with Windows, insisted on doing an erase and install when moving from 10.5 to 10.6. There is no discernible difference in their behavior. However, it took me under an hour to upgrade this machine to 10.6. It took him most of the day to back up, do the upgrade, then do the restore. It's your time, and your choice.
I am in the same spot. I just purchased an iPhone 4, just to find that I need to upgrade from tiger. I am pissed because the Apple store said that I needed to buy the $169 box to upgrade. Will the $29 snow leopard upgrade get me into iTunes so I can synch? That is all I care about.
Here's what I just found at Apple Store under the promo for Snow Leopard:
"Upgrade your Mac experience. With Snow Leopard, iLife '09, and iWork '09 all in one box, the Mac Box Set is the best way to upgrade your Mac experience, especially if you're still using Mac OS X Tiger."
So, unless it's a misprint (doubtful, especially at this late a date), it looks like a no-brainer. The may requirement is that you have an Apple machine with Intel processor instead of PowerPC.