I need advice on a third monitor : already have dual head setup

I just purchased a second Dell 2707WFP 27" monitor off of ebay. I've got an 8800GT with the usual dual DVI connectors, and I'll run one monitor on each.

Now, I have an older 22" monitor that currently serves as my secondary display. I could sell it on ebay, but I'm considering hooking it up so that I have a total of three monitors. How practical is this? Could I purchase a used PCI graphics card, plug it in, and get XP to use it?

I'm a total space hog, and 2 27" monitors at 1920x1200 isn't quite enough space. I'd like to have a whole monitor to stash various toolbars and widgets and control buttons.

But if there's not a stable way to hook up 3 monitors, then I'll just ebay this undersized 22" off and stick with just 2.