I want to updgrade the computer I have.?

Is it possible for me to upgrade my graphics card in my PC. these are the specs: advent CPD1301; AMD Dual-core II X2 545 3.0Ghz, 6mb L3 cache, 4GB ram, ATI Radeon HD 4650 1gb, 2 pci slots, 500gb HDD. im not sure about my power or anyhing. any help appreciated. thanks.