The E4500 uses socket LGA775: ARK | Intel® Core™2 Duo Processor E4500 (2M Cache, 2.20 GHz, 800 MHz FSB)
Processors that are compatible with the 945G chipset are: ARK | Intel® 945G Express Chipset (Intel® 82945G Memory Controller)
Processors that are compatible with the LGA775 socket are: ARK | Processor Feature Filter
However, it is not as simple as selecting a processor that fits your socket and is compatible with your chipset. You must make certain that the HP bios also supports that processor.
From the HP website: Motherboard Specifications, P5LP-LE (Leonite2) | HP® Support it appears that the highest processor you can use is Core 2 Duo E6x00 (Conroe) up to E6700.
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