3 monitors

You'll need a 2nd graphics card for a 3rd (and/or 4th) monitor, or a dual-GPU card w/ quad-display, to run more than 2 monitors. That is, unless your motherboard happens to have on-board video. In which case, you could use it to output to a 3rd monitor.

I should also add, I honestly don't have a clue what you're talking about in reference to that USB graphics card extend... Can you provide a link to it or it's product information?

Google maybe?

Well the simple thing to ask would be does the op know if his psu can handle it? wouldn't the draw of sli'd cards even those cheap ones kill any normal 300-350w psu...his wattage may even be lower considering it is a p4  

you can run three monitors with

2 graphics cards

or 1 graphics card and usb gpu

or quad head gpu