The AMd Phenom II X2 555 is better becuase of the L3 Cache but you are loosing a core. The AMD Penom II X2 555 and the AMD Athlon II X3 450 has nearly the same specs expept the number of cores and the L3 Cache memeory. The AMD Phenom II X2 555 will be about 10% performance over the AMD Athlon II X3 450 in most of the benchmarks