How far could I overclock a Phenom II x4 965 BE or Phenom x6 1045 with a stock cooler and MX-4? My choice of CPU depends on this.
Overclocking on the stock cooler is not recommended for any system. I have pushed a few AMD CPU with that stock cooler 200MHz or so but eventually the heat will damage the CPU. The best option is to replace the stock cooler with a low cost heat sink as shown below.
I believe the stock cooler on the 965 is the same one as the one on the PhII X4 980. The 980 runs at 3.7GHZ. Watch your temps and you should be able to get to that speed with just the multiplier alone. And MX-4 is a better TIM than what's on the stock cooler.
+1. The stock AMD Coolers have a tendency to suck, especially for their higher TDP chips. For $20 (on sale) you can get a Hyper 212+, which is an AMAZING budget cooler. I have one on my 1100T, and it never gets over 46 degrees C, and idles from 28-32 C. (Ambient temps of 24 C). It's also a lot quieter than the stock cooler. I would hugely recommend buying one. Also, for gaming the X4 is probably going to be better, since it has an unlocked multiplier for easy overclocking (it should run fine at 4GHz- I can get my 1100T to 4Ghz, but I really don't need it.) Most games aren't very multi-threaded either (enough to use 6 cores well), which is a plus for the X4.