2 video cards vs one with same total price

I am a heavy gamer and in the new system that i am building, i wanted advice on video cards. I was thinking of getting 2 EVGA GeForce GTX 470 (Fermi) cards. I was looking into other cards and i was just wondering if i got one HIS Radeon HD 5970 (Hemlock) card instead, it would be better proformance. They equal the same price but would one card at that price be better than 2 that total that price? I want to know if i overclock the 2 cards and overclock the 1 if that would also make a difference. The resolution that i plan on using will be 1920x1080. Thanks!