I haven't used XP in quite some time, but I'm sure it won't be too far different than win 7 was.
1st, make sure to plug in the monitors to which ever card you want to drive each.
If you can find XP display page by right clicking the desktop and clicking display properties or screen resolution, you'll see an option to detect monitors. Click it and apply, then arange the monitors in the proper order.
If you have the Nvidia control panel installed you can make use of both. You need to open the display properties and "Extend my windows desktop onto this monitor. you can also in NCP use advanced mode make sure all montiors are plugged in and run display optimization wizard along with set up multiple displays. you should have all 4 hooked up to your pc using 2 GPU's. I hope this helps you.
Make sure that the slot is OK by trying one video card in the second slot. After all, every now and then you may get a defective board. That card should then operate with 8x lanes since it's a 790GX board. If a card does not work in the second slot, then you probably got a defective board.