Athlon 2 x3 435 overclock

What exactly is limiting you? At what level does your computer start acting funny?

hey comp I got the same PCU, I just bumped the bus speed up to 210 Mhz and it gives me 3.117Ghz. haven't had any issues yet been runnin like this for 5 or 6 days. I also found it really easy to get the fourth core running. Whats ur mobo??

Sorry it 215 on the bus speed...

If you have an ASRock Mobo, you can use the Auto Calibration (A.C.C.) in bios to unlock the 4th core, or D/L AMD overdrive to tweak your set up, I was running at 3.2GHz using that, stable as a table but it wouldnt go any higher, (had 2Gb of 1333 Ram on it at the time), Bus speed of 220 will give you about 3.190GHz, 230 gives 3335,drops the pci speed to 99 and it lowers the clock timings on your ram , but i loaded the 'optimum bios' setting in bios to get that and that was just this morning but everything seems fine so far, ill keep you posted tho  
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is my effort, learning as i go  

well, if you care enough with the temperature, I suggest you lower your core voltage too. Mine is runing on FSB 220 but I set the core temp to 1.372 (the default is 1.408) it does lower core temp about 6C. I tested it using Prime95 for 7 hours plus and Memtest86 for 5 hours plus and no problem at all. Then unlock the 4th core, and see what happens. If everything is OK, try to run the Prime and memtest again. If it is fine, then you can be pretty sure that your system is running fine.
mine is using stock HSF able to reach 26C min and 51C Max after OC and unlock (my ambient temp is 30C)