i wouldn't bother with the x-fi - the mobo you're looking at has the excellent realtek ALC 882M audio, which among other things includes dolby digital live for driving digital speakers. CPU usage is also extremely low.
also, considering the e6600 is so hard to get right now, if you end up deciding to go with, say, an e6400, a P965 chipset will probably give you better overclocks. with the e6600 stick with 975X though.