Intel® Pentium® D Processor with Dual Core Technology Up to 840 (3.2GHz,2X1M,800MHz FSB)

As you could see from the links I provided, latest BIOS A11 does support Pentium D upto 960

The Optiplex GX280 does not support any Dual Core, or Core 2 Duo processors, single core only.

Socket LGA775

You can use any of the following processors:

• Prescott P4 Kit, 520, 2.8Ghz, 800FSB, 1MB
• Prescott P4 64bit Kit, 521, 2.8Ghz, 800FSB, 1MB
• Prescott P4 Kit, 530, 3.0Ghz, 800FSB, 1MB
• Prescott P4 64bit Kit, 541, 3.2Ghz, 800FSB, 1MB
• Prescott P4 Kit, 540, 3.2Ghz, 800FSB, 1MB
• Prescott P4 Kit, 550, 3.4Ghz, 800FSB, 1MB
• Prescott P4 Kit, 550, 3.4Ghz, 800FSB, 1MB
• Prescott P4 Kit, 560, 3.6Ghz, 800FSB, 1MB
• Prescott P4 Kit, 570, 3.8Ghz, 800FSB, 1MB

Intel 865G chipset, Intel® Pentium® 4 processor with 800 MHz front side bus and Hyper-Threading support or 533 MHz front side bus (depending on processor) and 512K L2 cache or Celeron® processor with 400 MHz front side bus and 128K L2 cache