Why don't you just use an Intel chip, rather than looking for something that's as good as it. Because I hate to break it to you, because AMD doesn't have a chip that can compete with the 2600k, and their best chip only ties the 2500k while costing $50 more. Intel's next generation of chips, launching in a month, will just widen the gap further.
try this for some more info but the Core i3-2120 seems like a good choice for money v. power.
You also need to find out exactly what you need rather then what you want and then go from there.
What programs are you running? what operating system will you use? do you need it very power efficient or have low sound? do you play on a regular basis the latest 3D games? do you want to have more that one monitor? do you need gigabyte Ethernet or will 100mb network or less be ok?
there are many questions you will need honest answers to before selecting components for a build.