"The real truth is that after you go above $200, you are wasting your money. It doesn't matter if it's Intel or AMD, VERY FEW PEOPLE NEED A CPU THAT COSTS MORE THAN $200. " - BaronMatrix
The total cost of a computer system includes an operating system, case, power supply, HDD(s), optical drive, keyboard and mouse, monitor, motherboard and memory. So by the time you buy the CPU you've already spent about $500-600 excluding the CPU and mobo, which are the components that mainly determine performance. Let's call it $550 for argument's sake.
The i5-2500k can be had for around $225
A good P67 motherboard will cost at least $200, so the cost of an i5-2500k build:
$200+225+550=$975
The X4 945 is $150 for the 95W version, a little lower for the 125W.
A decent AMD mobo would be around $125,
You save about $150, for a total parts cost of $825
What looks like a 30% savings if you only consider the CPU/motherboard is only a 15% savings considering the entire system. That's one thing to consider.
What about two or three years down the road? Unless this is your first computer you already know that the system that is just adequate today will very quickly reach the point where you're wanting to replace or upgrade it.
For that $150 spent now, you may get another 2 or even 3 years out of the more expensive system, plus you get the benefit of having had a better system for those 4 or 5 years.
Spend 15% more. Have a better machine that lasts twice as long. To me that's not a hard decision.