Yea, on-board graphics or a basic graphics card will be fine for standard video playback and slideshows. My card runs a 21" LCD and a 50" projection without any problems.
If you do want to go for a dedicated sound card something like the
Creative Sound Blaster X-Fi Xtreme Audio 7.1 PCI Soundcard from ebuyer. A lot of PC Gamers I know who don't use USB have Sound Blaster cards installed and are really impressed with them.