Bilfr3d

In terms of graphics, you could try launching the game using openGL. See this link for details on that. Generally, OpenGL uses much less memory than the default DirectX renderer. Aside from that, it's unlikely that there's going to be a solution for the out of RAM issue - make sure as many of your apps are closed as possible, to free up as much memory as possible.
Considering the recommended amount of RAM for system's playing KSP is 4GB, and the listed minimum is only 3GB, you should be able to run it.
 
EDIT: I've noticed you haven't actually provided details of your system. Could we get those? Installed RAM, OS, CPU, GPU, et cetera.

Duplicate of http://forum.kerbalspaceprogram.com/index.php?/topic/131214-old-parts-sticking-around/
 
To answer your question:
If you are on Steam, uninstall the game completely and re-install it.
If you downloaded the game through the store, delete all local files and re-download and extract the game.
The issue comes from old part models still being present in your GameData folder, and (I'm assuming because of partID and entityID reasons) the older model is chosen to be displayed. You COULD remove the files yourself, saving a re-installation/download, but I'd recommend re-downloading it anyway just to make sure it goes properly.