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The Hushed Casket was founded in 2002 shortly after the launch of the XBOX console. Today the Hushed Casket is a thriving community of gamers, playing together and publishing news and stories that have appeared in major media properties like Newsweek, G4TV, USA Today, and Penny Arcade. Some game developers have even referenced our guides and news to support their gamers. We don't play Halo. We LAN Halo.

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There's not too much to say
There's not too much to say that hasn't been said already about the advantages of building it yourself. And you'll have plenty of help. Rap's suggestion about the $500 rig from Tom's Hardware would be a really good place to start, or even build the exact same rig. And newegg.com is definitely the place to go for most (actually ALL) of your parts.
One thing that deserves mention is that, when you build it yourself, not only is the quality of the parts better, but since you are the system builder, those manufacturer warranties are extended to you, and not Dell or Sony, etc. Most parts carry warranties that last much longer than warranties that come with a pre-built PC. RAM usually carries a lifetime warranty. Most Hard drives have a 5yr warranty. Even some video card manufacturers back their cards w/ a lifetime warranty (XFX is the one that comes to mind). You get the point. And you don't have to pay anything extra for it.
Take advantage of the resources you've got here at the THX community. I'm sure you'll get more advice than you ever thought you'd need, but you'll have a killer rig and a good amount of knowledge to show for it!