Which new i7? If you're referring to the yet to be released Sandy Bridge versions of the i7, then no you cannot upgrade as they will be on a totally new socket (socket 2011).
According to the Dell site, the XPS 8100 uses a i7 870 CPU which uses Socket 1156 (please correct me if im wrong). If you wish to keep your motherboard then the only CPU you can upgrade to is the i7 870K which is the same CPU just unlocked. And there are chances that your motherboard may not support it. Not worth the money in eyes!
If you wish to upgrade to a i7 9XX (socket 1366) or a i7 2600 (socket 1155) then you will need a new motherboard.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Misfits Machine!
OS
Windows 7 Professional
CPU
Intel i7 4790K
Motherboard
ASUS Maximus VII Hero
Memory
32 GB G.SKILL TridentX F3-2400C10D-GTX
Graphics Card(s)
EVGA GeForce GTX 980 Superclocked X2 SLI
Sound Card
Teac UD-501 External USB DAC/Bryston BHA-1 Headphone Amp
Monitor(s) Displays
BenQ BL3200PT
Screen Resolution
2560x1440
Hard Drives
Samsung 850 Pro Series 256GB SSD
Western Digital Red 3TB
I expect you mean to the new Sandy Bridge I7 processor. That would require a new 1155 socket motherboard and new memory. With a DELL you might as well buy a new computer.
In any case you should not do this now because there is a design flaw and recall by Intel on the motherboard chips that support the Sandy bridge processors.
Samsung 850 Pro 256GB (OS), Samsung 2x 128GB 840 Pro SSD in RAID0, 3x WD Blue 6Gb/s 1TB RAID0, WD 2TB Black external USB 3.0, 2TB WD20EARS Green external USB 3.0, 2x 500GB Seagate and 1 750 GB external USB, 1x 350GB external USB3
If you are overclocking, the Sandy Bridge seem to do quite a bit better than their predecessors. Since it is a DELL, you can't overclock in any case so I agree with you in this case.
Samsung 850 Pro 256GB (OS), Samsung 2x 128GB 840 Pro SSD in RAID0, 3x WD Blue 6Gb/s 1TB RAID0, WD 2TB Black external USB 3.0, 2TB WD20EARS Green external USB 3.0, 2x 500GB Seagate and 1 750 GB external USB, 1x 350GB external USB3
The "new" i7 (Sandy Bridge) uses Socket 1155 (incompatible), and a new chipset (H67 or P67). It's completely incompatible. You'd need a new motherboard. Dell still uses boards with proprietary features, so it wouldn't be a trivial transplant.