You don't mention what CPU you currently have. Difficult to say what an UPGRADE would be.
Supported models:
AMD Sempron Processors:
•AMD Sempron140 Processor (2.7-GHz, 1M L2 cache, 4000 GT/s System BUS)
AMD Athlon II X2 Dual-Core Processors:
•AMD Athlon II X2 215 Processor (2.7-GHz, 1M L2 cache, 4000 GT/s System BUS)
•AMD Athlon II X2 240 Processor (2.8-GHz, 2M L2 cache, 4000 GT/s System BUS)
•AMD Athlon II X2 245 Processor (2.9-GHz, 2M L2 cache, 4000 GT/s System BUS)
•AMD Athlon II X2 250 Processor (3.0-GHz, 2M L2 cache, 4000 GT/s System BUS)
AMD Athlon II X3 Triple-Core Processors:
•AMD Athlon II X3 425 Processor (2.7-GHz, 1.5M L2 cache, 4000 GT/s System BUS)
•AMD Athlon II X3 435 Processor (2.9-GHz, 1.5M L2 cache, 4000 GT/s System BUS)
AMD Athlon II X4 Quad-Core Processors:
•AMD Athlon II X4 620 Processor (2.6-GHz, 2M L2 cache, 4000 GT/s System BUS)
•AMD Athlon II X4 630 Processor (2.8-GHz, 2M L2 cache, 4000 GT/s System BUS)
AMD Phenom II X2 Dual-Core Processors:
•AMD Phenom II X2 545 Processor (3.0-GHz, 7M L2+L3 cache, 4000 GT/s System BUS)
•AMD Phenom II X2 550 Processor (3.1-GHz, 7M L2+L3 cache, 4000 GT/s System BUS)
AMD Phenom II X3 Triple-Core Processors:
•AMD Phenom II X3 720 Processor (2.8-GHz, 7.5M L2+L3 cache, 4000 GT/s System BUS)
•AMD Phenom II X3 740 Processor (3.0-GHz, 7.5M L2+L3 cache, 4000 GT/s System BUS)
AMD Phenom II X4 Quad-Core Processors:
•AMD Phenom II X4 820 Processor (2.8-GHz, 6M L2+L3 cache, 4000 GT/s System BUS)
•AMD Phenom II X4 925 Processor (2.8-GHz, 8M L2+L3 cache, 4000 GT/s System BUS)
•AMD Phenom II X4 945 Processor (3.0-GHz, 8M L2+L3 cache, 4000 GT/s System BUS)
•AMD Phenom II X4 B95 Processor (3.0-GHz, 8M L2+L3 cache, 4000 GT/s System BUS)
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c01926404