You can get more then 3Ghz with an Athlon II 620 x4 on stock Volt and stock cooling.(Better cooling helps always)
I use a GIGABYTE MA785GMT-UD2H Mobo myself and it's very stable with the settings here.
Stock settings
FSB=200
200x13=2.600Mhz CPU
200x10=2,000Mhz NB
200x10=2,000Mhz SB
200x8=1,600Mhz(PC12800) x6.66=1,333Mhz(PC10666)
OC Settings
FSB=250
250x13=3,250Mhz CPU
250x8=2,000Mhz NB
250x8=2,000Mhz SB
250x6.66=1,666Mhz Memory
This is all done stable on stock V-Core.
I've run 10 hours of Prime95 in Windows 7 x64 no problems.
Recomendations
Get PC12800 memory, it's made for 1,600Mhz so it's better overhead for OC.
Get a good aftermarket Heat Sync or do a proper Lapping of the Stock Sync.
Use good Heat Compound like Arctic Silver 5.