Иду готовлю ужин. - it sounds strange to me. It's like, you are walking and the same time you are preparing dinner. You are preparing while walking.

Пойду готовлю ужин. Here, the future mixes with the present. It's like, you are only going to go, but somehow you are preparing dinner now. It sounds strange, too.

Пойду готовить ужин. It's correct. It's like, you have strong intention to go to prepare dinner. "It's enough to delay! I'll go and prepare dinner. "
Пойду приготовлю ужин. - correct. You just decided to prepare dinner.