Some PCs have built-in amps that can drive regular 8 ohm stereo speakers with a fair amount of power. I just upgraded (more like replaced) a friend's E-machine PC that had this - no external amp or amplified speaker needed for loud music volume. I think the only other model I encountered like this was a Compaq.
Generally that is not the case, and you would need (or want) an external amplifier to drive the speakers. It is best to get a sound card with at least two outputs: One for little pc speakers located at the PC, and an additional line-out to go to your amp. For a card with decent signal to noise ratio (and relatively low price), I'd recommend a Turtle Beach Santa Cruz for about $50 (at newegg.com - I like them and order stuff often).
For a low-end amp you may want to consider one of the surround sound setups that have a DVD player with built-in five channel amp and some satellite speakers. These range from $150 to as much as you want to pay. (Wal-mart, Best Buy, Etc) Just make sure the DVD player/amp has a set of auxillary inputs. Some also have AM/FM recievers built-in.
