you need a good motherboard, otherwise it will slow the overall performance of the other components.
Try a MB with 1066 FSB
You also need a 16x pci-express port
altho im not sure if conroe are supported.
As for the sound card, just as a graphic card takes all the video processing off the main cpu, the sound card handles all the sound so the cpu doesnt have to, besides, good expensive ones have diferent inouts and even lcds displays so you can plug your guitar and start recording.
The normal user might not notice any diference, but you might notice it on games with many sounds playing at the same time.
Sound samplerate also increases (if you ever downloaded the same mp3 song at 56kbps and then at 128kbps you will know what im talking about) so given a good set of speakers, you will notice a much better sound.