I don't see anything about the Dell that would insprire me to pay more than double what you could build that Sandy Bridge system for. If you really wanted to upgrade while spending as much as that Dell, I would look into building an SB E system like the Double-stuff workstation found here; http://techreport.com/articles.x/22513/8
Considering you wouldn't need the graphics and probably not all the storage, you're looking around $2k for a system that would beat the dell and the other SB system in a few areas.
Keep your stated config but swap in an X79 mobo:http://www.newegg.com/Product/Product.a ... 6813131800
i7-3930k processor:http://www.newegg.com/Product/Product.a ... 6819116492
and I think you would be well served by getting an SSD in there to nicely balance things out. Probably don't need the 256GB from the system guide, so I would recommend the 128gb version here:http://www.newegg.com/Product/Product.a ... 6820147134
My newegg cart is showing around $1,8500 for everything, more than a grand less than the Dell and that includes a faster/better processor, more/faster RAM, and an SSD.