That looks like a good computer but I would change a few parts.
The i7 920 is great.
I would add an after market heatsink/cooler. Xigmatek or Noctua are the best performers I believe.
Asus P6t is a great motherboard.
I wouldnt use that memory. I dont have DDR3 and im not sure what your best bang for the buck would be there but that has a very high cas latency ( CL ) for DDR3-1333. I love Corsair but you can do better there. Hopefully someone else can chime in with a better kit?
The 5870 is a beast of a GFX card. There are shortages right now but I would wait for it. Sapphire is a great ATI brand.
I would change the case to an Antec 900. Much better cooling.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129021
Corsair 750TX PSU is great. I have it myself.
I would go with a Western Digital Black series hard drive myself. Or the new Samsung Spinpoint F3. Or a WD Velociraptor for max speed but more money. Newer and fastest of all solid state drives ( SSDs ) will also be fine. You are only limited by budget here.
DVD,mouse,sound and ethernet are all good. Add some thermal compound like Arctic Silver.
The GTX 260 is dominated by the 5870.
This machine would easily be able to reach ~4ghz overclock even with you being a newbie
Dell does not give you much if any room to overclock or even update parts. They want you to buy a new Dell. Not to mention all the crap they put on your hard drive....trial software and the like that you will not need or can do better than......free in most cases.
This would be upgradeable to the new 6 core Gulftown chips out early next year if you feel the need. And you could add a second 5870 in the future in Crossfire if necessary.
Edit: I could do better with a budget. Also what are you using it for?