I'd actually start by asking what his current "all out" 1366 based system can't/doesn't do that a new build will. Does he just have the upgrade/new build itch, or is it something else? There is no wrong answer, just curious. :P
To Fizzy: "Sound Card? I guess we stick with on-board?? Realtek ALC898" If this guy does "heavy audio recording work" he should have something like a RME Hammerfall DSP 9632 card for audio processing or something similar for audio input/output.
I built a i5-2600K/P67 based machine with a SeaSonic S12II 620, 8GB (4x2) G.Skill X Ripjaws and GTX 560 about a year ago for a buddy with a "serious" home studio (with the aforementioned RME card into a Yamaha MG32/14FX mixer console) about a year ago. So something similar with current parts should do just fine. It also handles video editing just fine too (for editing his bands shows/gigs).