Old PC, new PC - doesn't matter. This DisplayLink chip-equipped display adaptor from Sewell will take any ol' PC and provide support for resolutions up to 2048x1152, all powered via USB.
So yeah, if you're been chomping at the bit to get HD quality resolutions delivered to your monitor via a USB connection, this may be the adaptor for you. Comes in a kind of periwinkle blue hue for $US99. [Sewell via Engadget]