Any of the Core 2 Duo CPUs will support a 64-bit operating system.  This system will support up to a T7600 CPU.

T7500 won't work in the D820. That's a Santa Rosa CPU and uses a socket P instead of Socket M.

Any Core 2 Duo whose model number has an even number after the 7 will work with a D820.

If the 2nd digit is odd it WILL NOT WORK.

No T7300

No T7500

Because it's easy to put electrically-incompatible CPU into the socket 479 and thus damage the CPU and/or the motherboard, it is very important that you verify processor compatibility before buying and installing it.  800mhz FSB CPUs do not work in Socket M.

Intel® Core™2 Duo Processor T7200
(4M Cache, 2.00 GHz, 667 MHz FSB)
Socket M

Intel® Core™2 Duo Processor T7400
(4M Cache, 2.16 GHz, 667 MHz FSB)
Socket M

Intel® Core™2 Duo Processor T7600
(4M Cache, 2.33 GHz, 667 MHz FSB)
Socket M

Mobile Intel® 945PM Express Chipset
(Intel® 82945PM Memory Controller)

• Intel® Core™2 Duo Processor T5600 (2M Cache, 1.83 GHz, 667 MHz FSB)
• Intel® Core™2 Duo Processor T7200 (4M Cache, 2.00 GHz, 667 MHz FSB)
• Intel® Core™2 Duo Processor T7400 (4M Cache, 2.16 GHz, 667 MHz FSB)
• Intel® Core™2 Duo Processor T7600 (4M Cache, 2.33 GHz, 667 MHz FSB)

(PS: sorry, the range is up to T7600.)

well. for the D820 you need to use any of the intel T7200-T7500 core 2 duo processors.

The chipset type on your motherboard determines if you can use the T7000 series or not.

There are several chipsets that have been used for the D820, you can check it with an "intel chipset check software" (called something similar...) that you can download from intel.

What I do not know, is how it is with the drivers - you probably need a new windows 7 on the computer, that is 64 bit capable (and not a system with 32bit capability), and then you will also need new drivers for all components. I doubt that dell has them (for 64 bit) - but who knows?