hi all i have a friend with a pc which he is now going to use it as a sli gaming rig
his question is - he has an antec truepower 550 watt psu is it suitable for this sli rig he is going to build

I would guess yes.

Here is one example:

Antec TruePower 2.0 Power Supplies 550 Watt
This computer is SLI-Ready! The motherboard has two PCI-Express x16 slots, making it SLI-Ready. The Power Supply Unit is listed as being SLI-Ready on the Nvidia website.

http://www.computercraft.com/sale/messages/321.shtml

More links: http://www.google.com/search?q=sli%20gaming%20rig

I would think that the motherboard specs would carry the exact requirements.