According to Various benchmarks, your looking at a 40 - 50% performance difference clock for clock on single core operations.
As a Result your Phenom II X4 would have to be 4.8 - 5ghz to be equal to the Sandy bridge CPU @ Stock - these are synthetic benches and calculations though.
An Educated estimate based on various benchmarks.
You still lose because Sandy Bridge's architecture is still too good! I also have 955, we also lose to the i3-2100... Dual core. Intel is too good. Hence why i3-2100 is on Tomshardware's best gaming cpu list. It's really close to the i3-2100 but the i3 is SO much more efficient than an OC'd Phenom II beacuse it uses SO much less power.