I'm on a team thinking about porting an 360 game to PC, with more improvements ofc.

The problem I have is that we need a method to render/control hundreds of enemies and weapons at the same time without bigger impact on FPS. The idea is to do the game in 2/.5D space shooter. The old version manages to do it pretty well now, but we're greedy for more.

I read upon some methods, but I can't recall them now D:

You can see a video of the current version in here: http://www.youtube.com/watch?v=vLdkPus9rTA

use a single function to loop through them all? it is better, performance-wise, than using sperate actions for each of them.

I can only speak for OpenGL and I don't know if you are using it and/or if the techniques are also available for DirectX.

First: Render as much in one draw call as possible. Batch as much objects as possible in one vbo.
You should also order the objects so that you don't need to bind the texture every time. The best would be binding one texture once per frame, draw all objects that use the texture together and the bind the next texture etc.

You can also use two vbos for your objects. One for even frames and one for odds and then switch them every frame. Then change only the one that is currently not active and update the other one once you are switching them again. This makes sure that the CPU don't needs to wait for the GPU and you can use the CPU cycles otherwise.