Nope, not possible, since the Turion X2(dual core) uses a different socket S1(638 pins) compared to the single core Turion64 which is Socket 754(754 pins). So, if you want to go dual core, you"ll have to get a new computer. Consider the dv9000 which is the dual core update to the dv8000 series. It is not available yet, but should be out soon...
http://www.shopping.hp.com/web...ping...TOPROMO_DV9000ZEDIT: Seems like HP just released the dv9000z...
http://www.shopping.hp.com/web...ping...=computer_store
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HP Pavilion TX2500..AMD Turion X2 Ultra ZM-86, 3GB DDR2 800, 320GB 5400rpm SATA, ATi Radeon HD3200/M780G.