It gives you the specs...dimensions, connectors, pin-outs and of course you'll need a much more robust PSU for your system...looks like a MINIMUM of 400W with 27A and you definitely need more than the minimum.
Choose carefully . . it's the heart of your system.
I have and HP Pavilion a1650y series
Pentium 4 3.06GHz
2 GB DDR2-533MHz dual channel SDRAM
128MB NVIDIA GeForce 7300LE, w/ TurboCache (but planning on upgrading)
I will need a new power supply for the new graphics card I want to get
A GeForce 7900GS. Can I buy any PSU that fits that case and has the right connections, or do HP motherboard require specific PSUs.