Radeon 7000 won't install as 2nd graphics card to HD2400Pro

I have two monitors off a HD2400Pro and I want to install a third monitor on my system. As I have unused PCI slots I assumed the best way to do this was install an old 7000 series PCI I had to run the third monitor. I installed the card and on reboot Windows recognizes a Radeon 7000 series and reboots but as Windows is restarting the XP splash screen pauses for a second and the system just restarts again, now requiring safe mode or last known good config to proceed (which of course does not properly install the 7000). When I remove the 7000 everything's AOK and the system functions normally.

I've updated the motherboard BIOS to the latest version, set the BIOS to initilise the PCI-E or PCI graphics first and I even moved the card into every PCI slot systematically to see if that would work, but it achieves the same result. I have verified that the PCI card works AOK in another PC.

My system specs are:
Processor: Intel Core2 Duo E8400
Mainboard: Gigabyte GA-P35-S3G with 1x PCI-E 1, 1x PCI-E 16 and 5x PCI slots (no onboard VGA)
Main GPU: Sapphire Radeon HD2400 PRO
PSU: Seasonic 12v 500w,
OS: Win XP SP2

I'm currently using the ATi drivers that were installed with the HD2400Pro (8.512.0.0).

I hope someone can help me or advise me about a new PCI card assuming it's just a case that the 7000 can't work with drivers for the HD2400 installed.

Thanks in advance.