Hi and welcome to ExTS
Although your chosen PSU is just about enough to run that card I would consider upgrading that a bit [ the PSU ] ( if you can afford it ) to future-proof.
Found this - your card cheaper
click here
[not the black edition]
You will like this
click here
The "Boost" actually over-clocks the Video Card in-game from what I have read.
Gives that little extra "boost" during gaming.
The card in all other ways - is basically the same as your chosen one.
You may want to price this one up.
click here
This card ( 7950 ) requires 2 x 6 pin power connectors
Does your PSU have these ?
EDIT - just checked ......yes :)
It is a dual-slot card ( wide ) and takes up two slots
This is common with high-end cards.
Your PSU states 85% efficient.
This is normal for PSUs ,,,,,,,,,,,,,,,,your 550 Watts will be nearer to 540 Watts
This will get less as the PSU ages.