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.