hi all i need some help on the gaming front

i have a gigabyte 6800gt gv-nx68t256d-b card which i am trying to run in sli mode for advanced gaming my problem is trying to locate another gigabyte card the same i have been on ebay for weeks looking but so far no luck
if somebody has one for sale please email me but i think this may be no joy on that front
so any way can i run another acrd along side the gigabyte of another make or model as i have been told that they both have to be identical

if i can run another slightly different card

which card?
how to do it?
is it worth the effort?

i have an asus a8n-sli mobo with 2 gig of ram i know i will have to change my psu as well as at the moment i have a antec true power 550 watt unit but it aint for sli i think

any help or comments on the whole idea would be mosy gratefully recieved thanks spud

any ideas