The Pentium D 2.8ghz is not part of the Core family.
It's either a Pentium D 820 Smithfield or Pentium D 920 Presler CPU
Performance wise - these are the Passmark CPU benchmark scores:
Pentium D @ 2.80GHz = 735
Core2 Duo E6400 @ 2.13GHz = 1264
Core2 Duo E6550 @ 2.33GHz = 1441
Core2 Duo E6700 @ 2.66GHz = 1636
Pentium Dual-Core E6500 @ 2.93GHz = 2048
Core2 Quad Q6700 @ 2.66GHz = 3351
All are Socket 775 CPUs and the CPU voltage values seem to be in the same range:
Pentium D 820 Smithfield 2.80GHz Voltage: 1.25-1.4V
Pentium D 920 Presler 2.8GHz Voltage: 1.25-1.4V
Core2 Duo E6700 Voltage: 0.85V – 1.5V
Core2 Quad Q6700 Voltage: 0.85V – 1.5V
Pentium Dual-Core E6500 Voltage: 0.85V – 1.3625V
But you want to be very careful about what CPUs the board can actually support - probably through BIOS upgrades provided by HP.
--> For that reason I think you want to browse the HP support forums and see what others that own the same MB have done for CPU upgrades. Best way I know to find out what really works.