The HP Pavilion p6510y Desktop PC uses an ATX (a.k.a. PS2) Form Factor Power Supply.
Standard ATX12V Power Supply Design specifications:
150mm W x 86mm H x 140mm D
5.9" W x 3.4" H x 5.5" D
The only dimension that will vary with power supply capacity is the Depth.
Measure the depth of your current power supply and look at the depth dimension of the power supply you want to purchase to make sure it will fit in the HP Pavilion p6510y Desktop PC's case and not interfere with the cables connected to the back of the CD/DVD Optical drive.
Select from top tier brands like Antec, Corsair, Enermax, Seasonic, XFX.