Difference between QX6850 and QX9650?

The QX9650 is a better chip by any margin.

The QX6850 is a 65nm(hotter) 4 instructions per a cycle CPU.
The QX9650 is a 45nm 5 instructions per cycle CPU and has SSE4.

This means the QX9650 runs cooler and at the same speed is faster.

QX6850
http://processorfinder.intel.com/d [...] Spec=SLAFN

QX9650
http://processorfinder.intel.com/d [...] Spec=SLAN3

Overclocking (selecting multiplier and front side bus clock) is adjusted via the motherboard BIOS. Follow the instructions in your motherboard BIOS manual to make multiplier and clock adjustments.

The QX6850 and the QX9650 are Core 2 microarchitecture processors.

Macro-Fusion is the major difference between Core and Core 2 microarchitecture. Core and Core 2 processors have four front end decoders. Core 2 processors can fuse pairs of x86 instructions together in the predecode phase and send them through one front end decoder.
Thus macro-fusion allows the predecode phase to send to the decode phase a maximum of either:
- four normal x86 instructions per cycle, or
- three normal x86 instructions plus one macro-fused instruction, for a total of five x86 instructions per cycle. *

Both processors have SSE4
The QX9650 has SSE4 plus SSE4.1

The QX9650 would be considered the better processor because it is the latest evolution of the Core 2 microarchitecture.

* Reference: Inside the Machine by Jon Stokes (pg. 257)

Both of these chips are aimed squarely at overclockers, the QX6850 has 584 million transistors versus 820 million in the QX9650. I own a QX9650 and this is definetly the better of the two if you are going to be using air cooling as it runs much cooler the the QX6850. The unlocked multiplier makes for easy overclocking but in essence you still want to base most of your core speed on the bus. You can simply change the multi to 12 for example and get 3.996 with a standard 333Mhz bus. I have mine dynamically overclocked which I strongly recomend to save power and to save the working life of the cpu. Mine runs rock solid at 4.2Ghz on the standard cooler which might get updated anytime soon for a possible 4.5Ghz on air, rock solid, all day. Fingers crossed.

Cheers guys!

So I gather that 12 is the highest multiplier you can have, right? So if I just raised the multiplier to 12 in the bios that would give me 4GHz. Would I have to adjust memory frequency and voltage etc. or would changing the multiplier be the only thing I needed to do. This would be on the QX9650.

Thanks