If you decide to go with a new sound card, you can either go the easy route with this:
http://www.microcenter.com/product/353890/Sound_Blaster_X-Fi_Go!_Pro_External_Sound_Card
If you don't mind popping open the case you could get this:
http://www.microcenter.com/product/310207/Sound_Blaster_Audigy_SE_PCI_Sound_Card
And if you just want cheap as fuck, you could get this: (note, I've never used this brand before so no clue on quality)
http://www.microcenter.com/product/239854/6_Channel_PCI_Sound_Card
Those are both PCI cards. They fit into one of the three white slots on the right side of your picture, so make sure you have one free before buying a card. The one I linked to first just plugs into any open USB port, don't need to open the case.