http://h18000.www1.hp.com/products/quickspecs/12668_div/12668_div.HTML#Standard Features and Configurable Components
Intel Pentium dual-core Processors
Intel Pentium Dual-Core E2140 Processor (1.60-GHz, 1-MB L2 cache, 800-MHz FSB)
Intel Pentium Dual-Core E2160 Processor (1.80-GHz, 1-MB L2 cache, 800-MHz FSB)
Intel Pentium Dual-Core E2180 Processor (2.00-GHz, 1-MB L2 cache, 800-MHz FSB)
Intel Core 2 Duo Processors
Intel Core 2 Duo E4300 Processor (1.80-GHz, 2-MB L2 cache, 800-MHz FSB)
Intel Core 2 Duo E4500 Processor (2.20-GHz, 2-MB L2 cache, 800-MHz FSB)
Intel Core 2 Duo E6300 Processor (1.86-GHz, 2-MB L2 cache, 1066-MHz FSB)
Intel Core 2 Duo E6320 Processor (1.86-GHz, 4-MB L2 cache, 1066-MHz FSB)
Intel Core 2 Duo E6400 Processor (2.13-GHz, 2-MB L2 cache, 1066-MHz FSB)
Intel Core 2 Duo E6420 Processor (2.13-GHz, 4-MB L2 cache, 1066-MHz FSB)
Intel Core 2 Duo E6600 Processor (2.40-GHz, 4-MB L2 cache, 1066-MHz FSB)
Intel Core 2 Duo E6700 Processor (2.66-GHz, 4-MB L2 cache, 1066-MHz FSB)
those are the only things worth upgrading to, and the problem is their all overpriced as crap for the performance compared to newer cpus