Pre fab or buy a custom built

[acmf]

I'm looking to upgrade my current PC. It will be primaily used for gaming and music. I am trying to decide a pre fab gaming PC (Predator,, Ebay) or a Dell XPS 9000 with a i7 chip.

The ebay one has:

CPU Intel Core i7 920 Quad-Core Socket LGA1366, 2.66Ghz, 4.8GT/s FSB, 8MB L3 Cache
Overclocked
i7 920 overclocked to 3.8 GHz*
Memory PQI 6GB (3X2G) DDR3-1333
Motherboard MSI X58 Pro Motherboard
Hard Drive 1 500GB 7200RPM 32MB Cache Serial ATA II
Optical Drive 22X DVD RW + Dual Layer SATA w/ Lightscribe Technology
Video ATI Radeon 4670 PCI Express, 1GB, DVI, VGA & HDTV Out
Audio 8 Channel High Definition Audio
Network Card Gigabit LAN controller
Ports 6 USB 2.0 Ports, 1x IEEE Firewire
Case CoolerMaster Elite 310 Mid Tower Case
Power Supply 575W Heavy Duty Power Supply
Warranty 3 Years Labor, 2 Year Parts Warranty & Life Time Toll Free


Any suggestions?

and will I be able to upgrade a Dell a few years down the road?

Thanks