The amplifier is what takes a low wattage signal and amplifies it to a higher wattage signal to drive your speakers.
A reciever has the amplifier built in so it has speaker outputs.
A pre-pro like the 970 requires a seperate amplifier (thus pre-pro/amplifier combonations are referred to as seperates).
The pre-pro or reciever also provides other functions such as source selection,volume control, converting digital signals supplied by a sourcs like your computer to analog so the amplifier can use it to drive the speakers, and bass management ( seperating the higher frequency sound that is sent to the speakers fro the low frequencies that are sent to the sub-woofer, and applying other audio and perhaps video processing functions.
The sound card in you computer can serve many of the same functions as a pre-pro but still requires an external amplifier and only controls signals from the computer so in most cases, it cannot be used to feed your tv audio or signals from other devices to the speakers.
Another option is powered speakers. in this case, the amp is built int the speaker, so the signal from the pre-pro or sound card goes directly to the speaker to be processed.This is how computer speakers work and some are made to connect to the variable audio output of your tv so the tv becomes you source switching device.This is kind of a dead-end solution though as there isn't a logical upgrade path should you be intrested in improving you sound quality in the future.
It all kind of boils down to the quality of sound and flexibility that you expect from your system now and may wish to attain in the future.
I hope this helps a littel.
Above all have fun and enjoy the music.