yes you MUST connect to both sockets , your card will underperform and cause you all sorts of problems if you dont.
you say your not in possetion of the psu yet , have you bought a new one?
you must be very carefull when buying a psu for a high'ish end system like that , first of all as your going crossfire you need at the very least 2 x 12volt rails with 18 amps minimum on each rail . if its a single 12 volt rail you'll need 30 amps minimum any less and your rig will suffer , it will underperform at the very best and at worst it'll crash all the time.
if its a crossfire certified one you'll have all the required connectors , if not it doesnt matter as lon as its a good psu.
550w is ok , its pushing it , but its ok
what make is it?
if you goto overclockes you'l see the Sapphire ATI Radeon X1950 XT-X TOXIC Master Edition 512MB GDDR4 AVIVO Dual DVI (PCI-Express) - and the Sapphire ATI Radeon X1950 XT-X TOXIC Slave Edition 512MB GDDR4 AVIVO Dual DVI (PCI-Express)
ive built a system for a friend with these and they are some serious kit :0) both the same price