It is not compatible:

Dell-supported processors for PowerEdge 840:

Single Quad-Core Intel® Xeon® 3200 series
Single Dual-Core Intel® Xeon® X3200 series*
Single Dual-Core Intel® Xeon® 3000 series
Single Intel® Core™ 2 Duo E4000 series
Single Intel® Pentium® Dual-Core 2100 series
Single Intel® Celeron® 430, 1.8GHz, 800MHz front side bus, 512K cache
Single Intel® Core™ 2 Duo E4700
Single Intel® Pentium® Dual-Core 2220
Single Intel® Celeron 1200

* The PE840 system launched before type "II" is not supported with Xeon CPU X3200 series.

Intel supported processors for the Intel 3000 Chipset:

Intel® Celeron® Processor 420 (512K Cache, 1.60 GHz, 800 MHz FSB)
Intel® Celeron® Processor 430 (512K Cache, 1.80 GHz, 800 MHz FSB)
Intel® Celeron® Processor 440 (512K Cache, 2.00 GHz, 800 MHz FSB)
Intel® Celeron® Processor 450 (512K Cache, 2.20 GHz, 800 MHz FSB)
Intel® Core™2 Duo Processor E4300 (2M Cache, 1.80 GHz, 800 MHz FSB)
Intel® Core™2 Duo Processor E6400 (2M Cache, 2.13 GHz, 1066 MHz FSB)
Intel® Xeon® Processor 3040 (2M Cache, 1.86 GHz, 1066 MHz FSB)
Intel® Xeon® Processor 3050 (2M Cache, 2.13 GHz, 1066 MHz FSB)
Intel® Xeon® Processor 3060 (4M Cache, 2.40 GHz, 1066 MHz FSB)
Intel® Xeon® Processor 3070 (4M Cache, 2.66 GHz, 1066 MHz FSB)
Intel® Xeon® Processor X3230 (8M Cache, 2.66 GHz, 1066 MHz FSB)
Intel® Xeon® Processor X3210 (8M Cache, 2.13 GHz, 1066 MHz FSB)
Intel® Xeon® Processor X3220 (8M Cache, 2.40 GHz, 1066 MHz FSB)