My PII X4 955 is overclocked to 3.5GHz @ 1.367V.
Every CPU is different. If your chip is on the C2 stepping (rather than C3), you can expect lower overclocking headroom and will have to adjust the voltage more aggressively.
Once you've found a good (stable) overclock, it's wise to set those directly in the BIOS, rather than use AMD Overdrive.
Desktop: AMD Phenom II X4 965 @ 3.5GHz, Sapphire Vapor-X HD5870, ASUS M4A78T-E, 8GB Kingston/Mushkin DDR3-1333, 256GB OCZ Vertex 4, 2xSeagate Momentus XT 500GB Hybrid Drive (RAID0), ASUS Xonar Xense, Corsair HX750W, Antec Nine Hundred, Windows 7 Professional x64.
HP Touchsmart TM2: Intel Core i5 430UM, Hitachi 250GB 7200RPM HDD, ATI Radeon HD5450, 8GB Samsung DDR3-1333 (CL9), Windows 7 Professional x64.