PCIe reduces PCI performance?

Hello, I am running a 3 monitor setup using a PCIe (GeForce 9800 GT) card for two monitors and a PCI (GeForce 8400 GS) card for the other. I've been able to setup both cards and they seem like they're installed correctly, but the performance on the monitor hooked up to the 8400 GS card is terrible. The colors seemingly display fine, but the monitor seems to really lag when trying to perform simple tasks like handling jquery (javascript) effects on a webpage or trying to play a video. The video just stutters.

If I switch that monitor to the other card it displays fine so it's definitely the 8400. The thing is, it doesn't seem like the card should be performing like this, does anyone know what's wrong? Is this a driver issue or did I get a bad card? Any help?

Not familiar with NVidia cards, but was thinking about doing something similar using 2 ATI cards I have. One thing I was told to consider was my power supply, as both cards require the 6-pin power connection - and depending on the PSU W it may struggle to power both cards adequately. It could be that is the cause of your problem, with the 8400 not getting enough power?!? Might be worth checking...