Okay... from a FAQ about my machine:
Does the 400SC have an AGP slot?
A: Of course! (The AGP slot is, however, not supported by Dell.) Picture here and diagram here. The AGP socket is keyed for universal 0.8v AGP cards that comply with the AGP 3.0 specification or 1.5v AGP cards that comply with the AGP 2.0 specification. The AGP connector supports 8x, 4x AGP 3.0 add-in cards operating at 0.8v, or AGP 2.0 add-in cards operating at 1.5V. There is no support for AGP 2x operation or legacy 3.3V AGP cards.
So, looks like if I want dual head, this is what I need to get. I'm so out of touch with video card interfaces, etc. Does anyone have a specific, inexpensive card they'd recommend? I'm not a PC gamer, and don't do anything overly taxing with this machine other than remote access to work, net stuff, etc.