You could just get a 9800GX2 and do a 3-way SLI. I don't know if nVidia allows that though.
Don't think that's possible. One 9800GX2 is actually two 8800GTS 512MB in SLI, and with nVidia you have to SLI the exact same GPUs.
As for the OP...
The 9800GX2 has two DVI ports and one HDMI port. I suggest you do what jyjjy said, either buy another 9800GT for SLI, or sell your current 9800GT and get another card - an HD 5770 would be a good choice with DX11, Eyefinity etc.
Little side note - the only card I know of that has four DVI ports would be the HD4850X2.