I Would Like To Upgrade My Cpu.i have a dim.E521 w/amd 3800+ cpu. how for can i go and not have to change the power supply?what are the specs.for a new cpu?

Dell Dimension E521
Processor : AMD Athlon 64 X2 3800+ @ 2000 MHz
Physical Memory : 3.5 Ram DDR2-SDRAM
Video Card : Nvidia Corp GeForce 8500gt
Hard Disk : ST3160812AS (160 GB)
Hard Disk : WDC (250 GB)
DVD-Rom Drive : TSSTcorp CDRWDVD TS-H493A
Monitor Type : DELL E207WFP LCD 20' MONITOR
Operating System : Microsoft Windows Vista Ultimate
DirectX : Version 9.0c