Firstly is it Basswood or Buckeye?
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&docname=c00748726
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&docname=c00783637
Regardless, you can only change your processor to one of these:
Basswood:
Intel Core 2 Duo E6x00 with Dual Core technology (Conroe)
Pentium D 9x0 series Dual Core (Presler)
Pentium D 8xx series (Smithfield)
Pentium 4 6x1 series (Cedar Mill)
Pentium 4 6xx (Prescott)
Pentium 4 5xx (Prescott)
Pentium 4 LGA-775 HT (Prescott)
Buckeye:
Socket: 775
Supports the following processors:
Intel Core 2 Duo E6x00 with Dual Core technology (Conroe core)
Pentium D 9x0 series Dual Core (Presler)
Pentium D 8xx series (Smithfield)
Pentium 4 6x1 series (Cedar Mill core)
Pentium 4 6xx series (Prescott core)
Pentium 4 5xx series (Prescott core)
Pentium 4 LGA-775 HT (Prescott core)