If you're planning on overclocking, definitely get that HSF. Yes, you can use your current hard drive. But note that you would have to reinstall Windows, because if you put your current hard drive, in with that computer, it won't be recognized. This is because, your current hard drive, is tied to your Dell's motherboard. Onboard sound these days are pretty decent. But seeing as you love music, I think this will be good enough:
Newegg.com - Creative Sound Blaster X-Fi Xtreme Audio 7.1 Channels 24-bit 96KHz PCI Interface Sound Card - Retail
or
Newegg.com - Creative Sound Blaster Audigy SE 7.1 Channels 24-bit 96KHz PCI Interface Sound Card - Retail
...but don't take my word for it, as I'm not familiar with what is a good sound card.
Ouch, pricey. Okay so I think the guy can handle reinstalling windows for me. And I guess I'll just use my current sound card (with SRS Audio Sandbow, you'd think it was pro).
Just in case I can't do this, was that $600 computer up there good? Could it even handle what this computer would?