Got an x2 550 unlocked to x4 B50 that has been running stable at ~3,5ghz since release. pretty happy with the bang for buck on that one 🙂 not tried doing anything with voltage though, maybe it'll go higher?
That's an excellent unlock and overclock for stock voltages. You should overclock the CPU-NB a bit so you aren't bottlenecked by the memory controller and L3 speeds.