When X2 was released, it even beats the previous FX series like FX 55 nd 57 in older games itself like FarCry, HL2, UT04 etc.
Todays dual core doesn't need dual core optimised applicatons for gud perfomance. They share the load b/w the cores.
And dual cores being such affordable that we can get a dual core proccy for as low as 2.5k(lowest), why think bout single core?