Can I still get the Golden God! achievement in Wrath of the Lamb or will I only get it when I get the Platinum God! too?
The description of both achievements is: "100% of the game" but Golden God! was available before Wrath of the Lamb.
Yes. I had not yet achieved Golden God when I upgraded to Wrath of the Lamb, but after finishing the original game achievements I did receive it. I did not have to finish all the Wrath of the Lamb achievements.