I deleted a previous message which did nothing to answer the question but which presumably implied that you should have searched first. Agreed, you'd have found a lot by searching, but it doesn't take much to answer your questions.
If you wait for Snow Leopard to be released, you'll not have to pay for an upgrade. If you buy a new iMac that day, it'll almost undoubtedly come with Leopard installed and a free Snow Leopard upgrade disk. If you wait a few more weeks (the exact time depends on the time it takes to sell the Leopard-installed iMacs), you won't need to install the new OS.
If you buy an iMac with Leopard (i.e., between now and when they ship with SL installed), you do not need to upgrade, and in fact it might be prudent to wait for a few months to do so, depending on how stable SL is upon release.
If you decide/need to upgrade, it is almost always a very simple process, and you will almost never lose any files but might find some apps/add-ons no longer functional. As always, it's a fantastic idea to back up your system regularly and particularly before an OS upgrade, but, unless you're quite unlucky, you should be able to insert the disk, and an hour or so later (maybe a lot less, maybe a little more) have a new OS with your old files intact. It really is as easy as inserting the disk, following a very few instructions, and having a drink (well, away from the Mac) as you wait.
If I were making the decision as to whether to wait, it'd be based upon how much I wanted a new system. The SL release is months away. I am a huge fan of Leopard, so I don't feel as though I need to wait for SL to get what I "want" out of the OS. I'd rather have all the time between now and then to learn the OS (SL will by 99% the same as Leopard from a usability point of view) and enjoy the Mac experience.
