Reliable psu calculator, or just some advice

Hi there,

I'm currently thinking of selling off my xfx 7850, and upgrading to a Sapphire 7870XT. I've only recently upgrading my psu and I want to check if I need to upgrade it again to accommodate this new gpu.

I've tried a few of the power consumption calculators available online but they all seem to give me conflicting advice, can anybody recommend the most dependable one?

Here's everything I've got inside my case;

i5 3570k
Asrock z77 Extreme6
xfx 7850 2gb (single fan)
128gb SSD
500gb HDD
1 x dvd/cd rw combo drive
1 zalaman cnps5x cpu cooler
3 x 120mm case fans
1 x 10mm cathode
1 x usb 3.0
1 x usb 2.0

I'm currently using a 550w psu.

Thanks for your help in advance