I have a dell vostro 200 slim desktop, with a 8600GT card. That woks fine and runs my main display and my LCD TV. I wanted to run a 3rd display 1280x1024 monitor which I could use for watching videos on while working so I wouldn't have to turn on my 50inch LCD TV.
I tried using the on-board graphics together with the 8600GT but the o-nboard graphics get disabled with the PCI-E 8600GT plugged in. So I bought a PCI 8400GS 512MB graphics card. From what I have seen that is the most up to date PCI graphics card available however the performance is terrible, scrolling down a web page displays bad redraw and the same goes for video especially full screen.
The drivers that are installed are the same that are installed for the 8600GT.
I don't want to messa round with changing too much if it's going to be in vein and that I must just accept that PCIx1 is just really slow. I'm just not too sure though why the performance could be so bad.
try running just the PCI card and see what performance you get out of it by itself with 1 monitor. I suspect you have way to much going on for the processor and its choking on it. The cards are comparable in performance the 8400gs actually has a higher fill rate, but the bus handicaps it. You are shooting for the moon with a submarine.....