Quick Question - Will this work?!

[Seriphia]

Will this power supply work in my girlfriends system? She's wanting to use a Radeon HD 6870 that her friend gave her, but I'm not 100% sure that everything will hook up and work correctly in her prebuilt.

The Power Supply

Her PC is a "HP P7-1234"

 

[Dark Lord of Tech]

No it's too long.

3.39" x 5.91" x 6.30" (H x W x D)

 

[Dark Lord of Tech]

This one will , and it's the same size as the stock one and more than enough power.

Dimensions: 150mm x 140mm x 86mm (5.9 x 5.5 x 3.4 inches)

PCPartPicker part list / Price breakdown by merchant

Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($49.99 @ NCIX US)
Total: $49.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-12-06 12:09 EST-0500

 

[Seriphia]

This one will , and it's the same size as the stock one and more than enough power.

Dimensions: 150mm x 140mm x 86mm (5.9 x 5.5 x 3.4 inches)

PCPartPicker part list / Price breakdown by merchant

Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($49.99 @ NCIX US)
Total: $49.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-12-06 12:09 EST-0500

Will the power supply I listed fit in a P7-1254 instead? And how do you figure out these dimensions? That's the main problem I have with this stuff :/.

 

[Dark Lord of Tech]

Yes it's the same size as yours.

Hp P7-1254

Internal 300W (100V-240V)

Form factor: Internal ATX
Total wattage: 300W
Nominal input voltage range:
100-127V/6A (50-60Hz)
200-240V/3A (50-60Hz)
Dimensions: 150mm x 140mm x 86mm (5.9 x 5.5 x 3.4 inches)
This power supply has an LED to indicate a possible failure condition when LED is off and power is connected