dual monitors: pc specs, OS required, etc.?

You can use Windows 2000 but you'll want XP or later. Multi monitors never quite worked right on 2000.

As far as CPU/Mobo/graphics card...

Basically any videocard from ATI or NVidia with 2 VGA or DVI connections on it SHOULD work though I would get one thats been made since XP came out just so you don't have trouble finding drivers. Also, make sure its a DVI connector not DVI-D (or if it is that it will work with a VGA adapter). The pins inside look different from DVI to DVI-D and you can google a picture.

For your motherboard, well basically as long as it has the connections for your video card and CPU it should work no problem (barring software issues or the sky falling. Your CPU, once again if it will run XP it'll do fine. For slightly more responsive you could go dual-core CPU but its not necessary.

Finally, make sure your power has the connectors for your videocard. Older models pulled power through their connector to the mobo while many of the never require either a 4 or 6 pin cable of their own.

Dual Monitor Requirements

You just need a graphics card with dual outputs (most cards have this nowadays). If it's just for work purposes, a 512mb card will be fine. Motherboard slot type isn't particularly important, many older AGP cards also have dual outputs, they're just harder to find and more expensive.

You can also install an old-style PCI card in additional to your primary card, if your existing card only has one output and you don't want to replace it. Even older 256mb cards like the GeForce FX 5200 had dual outputs. But that's sort of a last resort, since PCI cards are all obsolete and such poor performers compared to any PCI-Express card.

There are even external devices which can provide dual screen support for laptops and computers with a single VGA output like the Matrox DualHead2Go. For laptops it's a viable option, for desktops it doesn't really make sense given the ease of replacing graphics cards.

Support for dual monitors goes all the way back to Windows 98. So if you're running XP or later, you're definitely fine. Note that when you first boot the machine, the BIOS messages etc may be duplicated on both monitors, or the 2nd may remain blank. But once Windows is loaded, the 2nd screen will be fully functional provided your display settings have it enabled and the desktop extended to it.

you'll just need a video card with more than 1 output ports. Most of the time those are the dedicated video cards and not the integrated ones. Then all you have to do is just plug monitors on the output ports it may be a DVI / VGA / HDMI / or mini HDMI port

you'll just need a video card with more than 1 output ports. Most of the time those are the dedicated video cards and not the integrated ones.