hi i want to know what is the best performance respect to the number of nexus to make a shooter game and run well, something like a half life , but no so well respect graphic details

Read the manual....the manual is your friend...

you don't need to increase nexus if your game runs fine with default 40. afaik it is basically a pre-allocated memory, which is a useless resource if you do not utilize it. increase when you need, can be set it for each level.