512MB 8400GS (much much) slower than integrated X300

I've just installed a PNY 512MB 8400GS PCI card in an Asus Pundit P1-PH1 barebones PC which has integrated Radeon Xpress 200 graphics. The aim was to give the graphics a performance boost for games my kids play (it's their PC), such as Rollercoaster Tycoon 3 and Zoo Tycoon 2 (so nothing too demanding on the graphics front).

Unfortunately the "upgrade" seems to have had the opposite effect. Video playback and graphics performance in general is much, much slower when using the 8400GS. I really don't understand it. There is no option in the BIOS to disable the onboard graphics, only to specify the graphics controller to use as the primary boot device. I've set this option to PCI rather than on-board and yet the monitor doesn't display the BIOS splash screen if the PCI card is hooked up, only showing graphics from this card when Windows 7 has started. Device manager shows the Radeon and NVidia devices and if I disable the on-board graphics in device manager then the PCI 8400GS output disappears.

Can anyone suggest what is going on here? The Nvidia driver I've loaded is the latest from their website.

I do have the latest 801 BIOS already, although there was a beta 901 subsequently. As for disabling the onboard graphics, I do believe it isn't possible from the research I've done :-(

The motherboard is integrated into the case - full specs for the P1-PH1 are here. As for what I've added to the barebones system:

2GB of DDR2 RAM
250GB SATA hard drive
IDE DVD writer
2.8Ghz Celeron D
PNY 512MB 8400GS PCI graphics card
Windows 7 32 bit

I was using it as an MCE2005 media centre very successfully with twin DVB-T cards for 3 years before retiring it earlier this year and giving it to the kids.