Old PC, new PC—doesn't matter. This DisplayLink chip-equipped display adapter from Sewell will take any ol' PC and provide support for resolutions up to 2,048 x 1,152, all powered via USB.
So yeah, if you've been chomping at the bit to get HD quality resolutions delivered to your monitor via a USB connection, this may be the adapter for you. Comes in a kind of periwinkle blue hue for $99. [Sewell via Engadget]