AMD Phenom II X4 Black 960T 

I think 4.2-ish could be had as X6 with reasonable voltage and run daily as well.

I currently have mine at 4122mhz @ 1.35v, and it tops out at 41C under load (240 rad, ek block, fans at idle).

Has a 50/50 chance of unlocking it to a Thuban (X6) processor and even if you don't you can overclock it pretty far thanks to the low TDP.

Paired this with a GIGABYTE GA-970A-UD3 which at stock/auto settings set my vcore to a ridiculous 1.44v. I knew this was far too high for 3.0ghz, so manually set things, currently have it running 8+hours stable of prime95 at 3.62ghz @ 1.28v. Knowing that I can go up to 1.5v while still being considered within safe limits, there is quite a lot of headroom.

And because it's a Thuban, you can run 4 DIMMs at 1600.

I personally recommend you that CPU if you have 4+1 mobo and you want to achieve higher clocks.

You'll need to think about VRM cooling and higher-end CPU cooling if you're planning on an unlocked OC.

Without unlock you can achieve 4GHz or more with lower voltages and lower heat issues.

Heat is an issue and I had to put a fan on my VRMs and move to an H100 to avoid throttling under full load.

The only downside between this processor and the i3-2100 is that you will have to OC this to about 3.7-3.8ghz to have equivalent core-for-core performance in gaming, but that's really not difficult with this processor and a half-decent motherboard.

Just because a board supports the hex-core Thubans out of the box doesn't necessarily mean it will the 960T.

I could not ask for better performance from a $125 price tag (I got mine on sale $10 off, would still buy it at $150).