New cheapest Quad Core Processor: AMD Athlon II X4 620

its review here.

Unclock your AMD Sempron 140 from single to dual cores

AMD Sempron 140 Sargas 2.7GHz 1MB L2 Cache Socket AM3 45W Single-Core Processor
Pros: Cheap, quiet, and dual core capable! Unlocked to Regor Athlon II X2 440 with a GIGABYTE GA-MA785GM-US2H board in a minute!