Take a little time to consider where you will be after you get a PC for use as an audio system.. How much money will you have spent? Will it be worth it in the long run, when you will be doing DIY going forward to upgrade and to reconfigure software and hardware. A computer salesman will not be up to speed on what you are trying to achieve, after you leave he will be selling another "perfect PC" to another customer he may never see again. Building a PC is no harder today than it was 10 years ago. Anyone can go into a Best Buy any week, and get a 250GB hardrive for $125 or less. Likewise anyone can follow the easy instructions to configure backups if they so desire.
There are nice power supplies and there are cheap pieces of junk. I just replaced a cheap piece of junk power supply with another cheap piece of junk power supply for $30, just to get a less-used PC at home back up and running. If I were building a music system I would get a very good power supply, something like an Antec $60-$100 at CompUSA (or you can order on-line for much less) and plenty of quality hard drive space. Nowadays, a terrabyte of storage inside one PC is not unheard of.
After you get a PC assembled and configured for you, you will be on your own anyway. I would decide how much it is worth it to me to have someone set up the PC to the point where I will be "done" (for a little while), versus getting more of a bare-bones PC (or even building it yourself) and moving forward with specific components and software I need for an audio system.
Selling PCs successfully today means selling the latest "sweet-spot" technology at a decent profit. For many applications the consumer doesn't need to do better, but you sure can do better. You will be working with the hardware and software going forward, more than with audio equipment probably.