The power supply only draws as much power as each part needs so if your PC parts use 300 watts total, for example, and the power supply is 600 watts then your PC would only draw 300 watts from the wall even though the Power Supply is rated for 600 watts. I looked up your PC spec sheet online and according to Dell it has an 1100 watt power supply. I would keep that PSU cause then you can add any graphics card you want or CPU assuming your case can fit it. Better to have a bigger PSU and not need it then need it and not have it. No need to worry about a massive electric bill. Your PC doesn't use much electricity anyway i don't think.