You need all the DVI/VGA/HDMI outputs on one graphics card, Windows does not allot the use of another video card for another display. You would need an ATi Eyefinity card to have three display outputs. I believe some nVidia cards allow that also.
Unless that PCI-E x1 slot is open ended then no, a graphics card (that are x16) won't physically fit in the slot.
Besides, you really need a x16 slot operating at a minimum bandwidth of x8 to get any form of performance benefit out of a second graphics card, especially if its for gaming.
My suggestion is selling whatever you have now and getting a bang up to date 6850/6870 that can support Eyefinity over three screens on just one card.
You will, however, have to have your monitors support at least one of the four major connection types across the 3 of them. Supporting a mix will be even better as the graphics card will need to output on a DVI connection, a HDMI connection and a DisplayPort connection.
I do have a PCI slot. My current graphics card is PCIE x16 nVidia 9400GT. I googled around and they recommended that if I use same graphics card model, which is PCIE 9400GT and PCI 9400GT then it will work coz it has the same driver. Is this true?