AMD's $75, 3.1GHz dual-core Athlon II X2 255 doesn't come with any onboard L3 cache. And its L2 cache (memory that's typically smaller, faster, and located closer to the core) is split into 1MB per core. As a 45-nanometer, Socket AM3 model, it's backward-compatible with any motherboard based on Socket AM3 or AM2+.
As for performance, when compared with AMD's top-shelf, six-core Phenom II X6 1090T, the X2 255 is only 15 percent slower on our general tests, but 70 percent slower on Cinebench, our multicore-focused measure. For another comparison, although the X2 255 runs at a higher clock speed than the quad-core, 2.9GHz X4635, the latter's two extra cores help it deliver an 8.9 percent increase in performance on our WorldBench 6 tests, and a whopping 83.6 percent increase on our Cinebench test.
This low-end CPU isn't that much slower than the Phenom II X6 1090T, except with multithreaded apps, where it's game over.