I got a buddy who wants to run 3 or 4 monitors on one PC. I've seen combo video card/dual lcd's online. Would two of these work in a PC (to give us 4 monitors)? What about using the onboard video in combination with one of these dual lcd setups (for three monitors)?

Any suggestions appreciated. TIA.

what would be the purpose?

This space for rent

He wants to trade stocks from one computer using real-time quotes.

I have three monitors running on my wife's computer. It's to run simultaneous instances of one program such as photoshop or quark. It helps her greatly when doing layering on picture development.
I have it set up with one pci-e gfx card with dual vga and a second pci card with dual vga. When I attempted to run one card with the on-board graphics I got conflicts galore and it often times wouldn't run any screen. Just make absolute certain that both cards you buy are of the same brand. Do not use an ATI card with an nVidia card or vise-versa. They do not need to be the same model. The PCI-E card she runs is an ati x300SE combined with an ati radeon 7000 on the regular PCI bus.