Try Newegg for this:
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E16883107984That is a quad core AMD processor, with tons of RAM and a 750GB hard drive. You can add a 500GB hard drive for about $70.
The power supply is 300 watt no name, and I would recommend upgrading that to something a little more powerful, say a 500 watt or something in that range.
No PCI slots, so if you have a PCI sound card, don't buy this. It has only PCI-e, on x16 for graphics and 2 or three x1 for everything else. Asus makes the Xonar D2X sound card which has PCI-e. There are really only three manufacturers of computer sound cards that support PCI-e, and that's Creative, Asus and AuzenTech, and AuzenTech still uses Creative chips. I like my Asus card, it offers 7.1, coaxial and optical digital ins and outs. It is expensive, however. If you're going to go Firewire <shudders> or USB <semi-shudder> then you should be pretty well off, as far as your audio interface goes.
If you're planning on gaming on this system, it'll support lower end games, WoW, Second Life, Sims, but if you want to run COD4, Crysis, etc., you're going to need to do a major upgrade to the video system.
Or, if you don't mind building your own, you can start here:
http://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.311917...and work your way down.
If you're handy with a screwdriver, and know your way around the inside of a computer, you might try something like this:
http://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.319352You'll still need some extras, like a CPU cooling fan:
I like Zalman.
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E16835118056You'll need a video card, but for audio only, and not gaming, that's not a huge issue:
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E16814102843Something like that would be a good, general video card, if you want to go blow up some people when you're not making music.
Finally, you'll need an OS.
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E16832116754Your choice on 32 bit or 64 bit, but 64 bit takes advantage of everything in that package.
Gary