Building a PC

[abecx]

The main reason to get a separate sound card is because of the the interference caused from having so many electrical components so close together. Also, it will offload the sound processing from your CPU. Honestly the vast majority of people do not need one and typically wouldn't notice a difference anyway.

Some motherboards now just separate the onboard sound on the mother board ( ie its attached to nothing else power wise and goes straight to the power supply ).

I would opt for the video card with more memory as well.