Well, the on-board video and sound usually or often work off the RAM on board (they borrow part of the RAM). If RAM's not an issue, most on board video and sound will work just fine for everyday tasks.
If I have video or sound cards available, I throw them in, but I've used on-board video with no problem (when I didn't mind the bite it takes out of your RAM) and used on-board sound with no problem at all.