I have an old Hp media center pc m7250n, POWER SUPPLY?

i have a hp media center m7250n desktop and the power supply is bad, i found the specific replacements online but they're way more expensive then buying that ones that arent the actual hp brand. but how do i find out what power supply will fit my pc? AND is there a way to upgrade a graphics card that will make it possible to play games like sims 3 and age of conan, star wars galaxies etc? this pc is pry 6 years old, so i am kinda worried i cant upgrade the graphics card