The gpu has a inbuilt in sound processor nvidia gpu's have a spdif input that allows for sound input for hdmi and ati uses onboard sound processing units. Tbh I would consider the onboard hdmi output fine if you want to improve later when you have better funds then get the asus xonar d2x or the auzentech high def card which hasn't been released yet, depends how impatient you are
Raid 5 adds data redundacy which means if one drive fails the you haven't lost the entirety of the data unless a second hdd fails within the time it takes you to order a new drive and "rebuild" the array. Samsung are the quietest drives and highly reliable company really can't fault.
As philbar said if you want true fast performance get a solid state disk that will be extreme, the raptors as of late from WD have been a bit dodgy it happens to all companies that they have a few months of dodgy products, imo OCZ make the best competitively priced ssd's. It only really makes a difference to start up times and perhaps your photoshop if your working from the os drive.