good morning to all

a few years ago i build a windows 98/xp retro gaming pc .
my specs are the following:

p4 3.0 northwood
Biostar P4VMA-M
512mb ram
nvidia fx 5500
creative live 5,1
120gb ide hard drive
lg dvd r/w
500watts psu (athlon xp capable)

my problem starts when i bought an nvidia 6600gt agp card to update the fx 5500

drivers were installed perfectly
windows reboots and.............
NVIDIA SENTRY immediatly reports not enough power to the card lowering the clocks to protect the system.
this on idle whithout any stress.

checking hardware monitor in bios , indeed i had 11.52 volts at 12volt rail and whithout any load.
i install back the fx 5500 and i connect my multimeter on 12v rail molex running 3dmark 2001.
the multimeter reports 11.98 volts on full power.

what is going on? why my mainboard reports 11.50volts instaed of 11,98 v and i cannot install a more powerfull card?

changing mainboard to another one does not solve the problem .

any help would be appreciated