Look, if you're thinking about getting an Apple TV and you think you'd use it and enjoy it then you should buy one now. If you can afford the $99 now, then you won't have any regrets if, by some chance, it's updated half a year from now. If you're so strapped for cash right now that $99 is an investment that you're worried will be outdated in six months, then you probably have better stuff to spend your money on, but that's up to you.
Regardless of any of that, nobody here really expects to see an update to the ATV any time soon. Certainly not by Christmas, probably not early next year, and most likely not until next spring/summer at the earliest, and even that's complete and total guesswork. Chances are you won't see an update for another year or so. If you want one, buy it now. And if the update to the ATV six months to a year from now is son incredibly awesome that you can't live without it, then buy that too.