No VIA Epia boards do 800x480, or any custom resolution. Your best bet is a Matrox, Radeon or GeForce PCI card, as most of those allow custom resolutions.
The other option is to just use 800x600, like many people do. It doesn't look too bad.
The screen you have bought is a Lilliput, please don't ever call it a Xenarc again, as it isn't.