The ASUS Xonar DX rules the sub-$100 space, more or less unopposed at this point. Huge improvement over onboard, assuming you have a halfway decent set of speakers.
As for volume, the voltage/current on the output port of more or less stable for every audio manufacturer, so its the speakers sensitivity that determines how loud they are. So don't expect a huge (if any) volume boost.
I don't have PCI-E on this :-/