I wouldn't ask the readers of a computer hardware enthusiast site whether you should buy a Dell or build your own. It's like asking the NRA if you should buy a gun or just stick with the baseball bat under your bed.
There are pro's and con's. The current deal for a 4700 + 2005FPW for $800'ish is almost unbeatable. Add $200-$300 for a bit more RAM and a PCIE video card (yes, the 4700 has a PCIE slot, and the option to upgrade to a 6800 at least), and you'd be set. You probably couldn't build a quieter, faster system without overclocking and getting some REALLY sweet hot deals on the individual pieces.
Don't let unspecified future "upgradeability" be a factor in your decision. Technology changes too fast to count on it. You may get lucky, but you probably won't unless you are upgrading every 9-12 months. I just upgraded from an Athlon XP to an Athlon 64 system, and the only items that worked in both were the HD and the DVDRW...not exactly a super cost savings, and my last upgrade was less than 2 years ago.
And everyone has left something out here...software. The Dell will come with XP and Office (of some version)...unless you have your own copies, that's $200 right there.
Personally, I've never bought a Dell or any other pre-built because I enjoy the build. Yes, building your own is satisfying, but opening up the box and having the whole system running 5 minutes later is also satisfying. You choose which is more your style.