try with stock voltage ( maybe +0.05 ) push until unstable .
my original heatsink ( after lapped ) can keep around 55°C load temperature close casing using 1.5V .
default Vcore should be around 1.35-1.4V for most s939 processors .
try to use manual BIOS setting not Overclock AI as you'll learn more especially the memory controller part .
ok , set everything to default and go download a small program called A64TWEAKER
write down these following value ( http://techreport.com/articles.x/9328/3 )
MEMCLOCK MODE limit
MEMCLOCK VALUE 166 or 133 if unstable
CL 2.5
TRAS 7 or 8 if unstable
TRP 3
TRCD 3
TRRD refer tweaker usually 2
TRC refer tweaker usually 10 or 11
TRFC refer tweaker usually higher than TRC by 2 , 3 or 4
TRWT refer tweaker
READ PREAMBLE auto
ASYC LATENCY auto
CMD 1
BANK
BURST
MEMORY HOLE
vdimm is voltage for RAM
to be honest , i don't expect high detail from a 8500GT , try lower the graphic detail .

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Added on December 8, 2007, 1:53 pm
according to google , your mobo support up to 1.55 for Vcore and 3.2 for Vdimm .
haha , K8 will never do 4.0GHz easily . some even struggle to reach over 3.0GHz .