|It's usually best to buy cards with the same video chipset brand - e.g. ATI or NVidia - then usually all custom settings are available in the one maker's configuration program.|
I've seen web sites on the web that show XP will support up to at least 8 displays.
Some mboards support 2, 3, or 4(?) PCI-E X 16 video cards, but they're usually more expensive and you're not likely to see that for a brand name system.
Brand name systems often come with power supplies with meagre output capacities. They will handle a modest video card, but not one that requires more power. You MAY have to upgrade the power supply.
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
In your case, the PS has to have enough capacity to handle both cards. PCI cards usually require less power than PCI-E X 16 cards. e.g. If one card requires a minimum 450 watt PS, another 400 watt, the PS would probably have enough capacity if it was at least a minimum 500 watts.
You can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this: