The XPS 8300 has an eSata port in the back which was designed to be quicker than USB 2.0 and specifically designed for external hard drives. I'm using mine with no problem for blu-ray. Judging by the photos and forum posts, it doesn't look like the XPS 8500 model has an eSata port. So another possibility is that you could get a PCI-e eSata card and connect an external hard drive to it. But if you want to go this route be warned, there's a 21 page thread that went on for 5 months complaining about eSata cards not working in the XPS 8500: http://en.community.dell.com/support-forums/desktop/f/3514/t/19449717.aspx?PageIndex=1 It appears they did eventually find a fix with a BIOS update but just be aware about this issue.
If you want an internal hard drive, another possibility is to get a SATA card (not eSata). The complaint above seemed to be mostly for eSata but there was a user who claimed his SATA card was working with no problem in the XPS 8500. It basically gives you extra SATA ports to work with. If you want to be more ambitious you could try using the mSATA slot, which comes in the XPS 8500. That's a completely different type of drive altogether (smaller and supposedly quicker). Be sure to read up on the forum posts about it because there seems to be debate about that topic as well. Good luck!