I recently purchased a Geforce FX 5200 PCI board with 64 MBytes of RAM. Something like that goes for $50-$70 on ebay. You just have to get one with two monitor outputs (e.g VGA and DVI). It is a secondary graphics card in my computer. When I set the AutoGen scenery to none, I get pretty decent frame rates even with all effects set to Max and textures to High (of course no Antialiasing).

Here is some screen shots showing the card operate in horizontal spanning mode playing Flight Simulator 2004. It essentially yields a single "virtual" 2048x768 display mode.

http://www.openacm.org/flight/index.html

I especially like the inside cockpit views at low zoom levels (nice wide field views), as well as the outside shots zoomed in where the plane takes most of the screens. Just beautiful!

Flight Simulator would be capable of running 4 of these graphics cards in spanning mode, with 8 monitors or LCDs attached. All Geforce FX cards have this spanning feature. ATI Radeons unfortunately don´t ;-(

I am going to run off and buy two identical LCD panels soon. This is just beautiful.