Dual monitors?, Please help! Options

[jiyong]

I'm trying to set up dual monitors for each of the computers in my dad's office (HP models m8226x, a6357c, & a6838f). We haven't made any modifications to the computers or changed the OS - the specs on each are still the same as on the HP site. The m8226x and a6357c both have the NVidia Geforce 6150 SE chipset and the a6838f has the Intel G33/G31 chipset.

I'm pretty much a noob when it comes to building computers, so I have no idea what I should do. I wouldn't even be doing the setup if it wasn't my dad asking. I do know that we'll need to buy some graphics cards, since there's only one VGA port on each computer. What kind should I buy? We don't need high-quality cards since nobody will be gaming on these computers (only data input, tax programs, scanning documents, and other office work), but we need to buy decent graphics cards at a preferably low cost. What sort of cards should I consider? Any help at all would be most appreciated (: