Aye, CPU was not my issue, it was my RAM getting fully used very very quickly on planets (5 to 10 minutes) and forcing the game to stutter to a stop or crash.
My hints are as follows :
* I believe the LOW Texture setting has bugs and a possible memory leak (about 1 Gb getting stuck in the virtual memory even after close).
EDIT : my theory is this was caused by the fact that my NVIDIA video card was imposing high quality textures while the in-game setting was imposing Low Texture. The incoherence between the two may explain why I suddenly had about 1.4 Gb of extra RAM in use. Yet, maybe not, 'cause not I have High Textures set in-game, but with NVIDIA Performance imposed. So, I don't know, but it works very well now.
* Maybe some settings set in game are not really applied, such as turning anti-aliasing off or turning vertical sync off. Setting these off in your video card preferences should do the trick.
Again, that is mostly RAM (as it was unplayable before with my 2.8 Gb real RAM). Have not experienced a grave problem with CPU, I just make sure the computer is well cooled/ventilated.
You sir, are a legend. I switched my textures from low to high after nearly tearing my hair out trying to figure out why the game kept crashing my PC due to filling up my 4GB of RAM. And now it's running perfectly - it's also solved the problems I was having with missing textures in the first place (enemies not showing up, them being funny colours, missing weapons, stuff like that).