It really depends. Simply on whether or not your applications can use more than 2 cores. Or if you plan on running many cpu intensive applications at one time.
The x2 is "faster" in apps that only use one or two cores. The x4 is faster in anything that uses more than 2 cores. I would go with the quad if i was you. But I wouldnt get the x4 920. I would jump up the x4 955.
By the way. I doubt your "freezes" is due to your hardware. Your machine even at FULL LOAD. should not freeze. Slow down, yes but it should not freeze.