AnandTech has put the lowest priced ($537US) X2 CPU to the overclocking table. At a stock 2.2GHz, they easily found themselves at 2.6GHz. Not an amazing benchmark, but not bad.
For an early x2 Athlon64 processor the overclocking performance on air is impressive. The next speed step of 2.4GHz (4600+) was easily reached, and we moved on past the next logical step of 2.6GHz to 2.7GHz. This is faster than any of the currently available Dual-Core Athlons. We should mention that 240×11 (2.64GHz) was an extremely easy and stable reach. That speed required just 1.45V, was exceptionally stable, and ran quite cool. The last 55MHz required a great deal more voltage and generated a lot more heat. For day in and day out the 240 setting (2.64GHz) would be our choice with this CPU (stock 2.2GHz) on air.
You can check out a lot more info and plenty of benchmarking results, over at AnandTech.