Increase the voltage to the AGP slot.
Also, pull it out and put the old video card in it's place, boot up, set ALL video settings to minimum settings, and try again with the 9800 Pro.
It may also be a driver issue, download and install the lastest drivers from ATI's website. And make sure you uninstall all of the previous card drivers.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w