production has stopped on both the phenom and AM3 mobos (bulldozer and AM3+ mobos are in production) so now would be the best time to purchase the phenom if you are in the market for one. i would still get an AM3+ mobo though for future upgrades (phenom II works fine in a AM3+ socket)
as for piledriver, im sure AMD will make some performance improvement (from bulldozer) but that really is not saying much
I have been building several rigs with the PhII X4 965BE. It still is a viable processor when O/C'd as mentioned above. And O/Cing it is usually as easy as simply increasing the CPU multiplier. You can turn it into a PhII X4 980 or better without it breaking a sweat. It's a $100 bargain. But if you want one, better hurry. They won't be around (new) much longer.
Too early to say exactly how much better PileDriver will be compared to Phenom II. AMD expects a 5% - 15% improvement for every new generation of CPU. A 10% guess-timate is reasonable. It's unknown what price a PileDriver CPU will be which has the same clockspeed of a Phenom II X4 965, but I know it will be more than $110.