If there will be ATV announced at the keynote on sept the 9th, it will be available not sooner than mid-2015 - so yes, you can safely buy an ATV now.
I just received today mine, and I not regret. Actually, the third generation has everything you need, and you'll enjoy every part of it.
Also don't forget that you can always sell your ATV near the premiere of the new generation.
I don't know where are u from, but try looking for ex-display ATV. I bought mine for (after conversion) about $70. - brand new, no scratches at all with seller warranty. (Although seller has mentioned that it may have few scratches). Lower price just becouse device was in a display case in the store