So how about this idea:

The Base level is out of a cell/region system, every region/cell leads to a file where all the Entitys are saved in - Position Action Behaviour stuff everything for this Region.

By leaving a Region the old one will be destroyed and the new one will be loaded.

I thought about this idea before, but the game still uses a lot of RAM even on small regions, because of the many entities. Thanks though
So, I'm a beginner in this part of game creation, how can I stick meshes together into one entity (i.e. chunk)?

Im a little bit confused... explain how your models work.

Are they big? Do they have high res textures? How many are they in a level?

They are small, about 32x32x32 quants size. They actually have high-res textures, with a size of 512x512. When I removed the texture completely from one of my cubes, it used a much smaller amount of memory, but removing the texture is not a solution
I tried one of my cubes at 16x16 texture size, but it didn't help much.
The problem is, the big amount of cubes I need in my level. If I build 3 houses, the game starts to lag, and uses about 600 MB of RAM. It's about a few hundred cubes, maybe 200-300 of them.

Have u tried using external Textures?

I have a Level with about 500 Modells in it wich are using 8 HiRes textures, thx to External textures i can add almost unlimited new models without losing fps or stronger memory.

Hires textures = 2048²

I haven't tried it yet.
Do you mean, to not apply the skin in the 3D modeller, but in source code? My models are pretty large because of the textures, my friend models them in 3ds Max.

Oh dear... U have to set this value in the Modeleditor, when creating a new skin and setting the Texture file, hitting the external Button.

I cannot explain that much also there is NO docu in the Handbook...

Hmm... I'll see about it. Thx!

to clear the texture thing up:

There is absolutely NO memory difference when using external textures. But usually it's better to use external textures, because they are easier to change and you can share the same texture between different models.

A texture that is stored internally requires more memory than the same texture externally when the external texture is compressed (pretty much every format other than .bmp or .dds)
Besides that, textures are never loaded uncompressed. Wether you use a 512² 84kb .png-texture or a 512² 1024kb .bmp-texture - it doesn't make any change in the memory usage because the texture is always converted to an uncompressed format (dds) when loaded by the engine.

The ONLY way to use compressed textures is using the .dds format with texture compression because dds is the same format as the textures that are stored in your vram. However, lossless compression is not possible.

"There is absolutely NO memory difference when using external textures. But usually it's better to use external textures, because they are easier to change and you can share the same texture between different models."

And thats essential, only with this u are able to load levels with much entitys fast enough. I think its also important for rendering, thinking about clonig rendering techniques.