i plan to buy 7970 and then add another in the future but my mainboard has pci 2.0 16x slots to close ...
mainboard === Asus p6t se
as you see gpus will be too close .. like 1-2mm space between them ... will it bring me overheating problems ? and if yes how to fix it ? i don't want to buy new mainboard ... is there any other ways ?
It's going to be better than what you see in the photo since your case will keep them spaced properly. Still, I'd get a fan or something to blow air into those spaces between the cards. Also definitely go with a blower design otherwise you'll be circulating hot air.