Portal Q&A

When i type sv_cheats,it unlock all the cheats,u know,but when i type in sv_gravity 0 or sv_gravity 600 or with a number or without a number,it says unknown commant : sv_gravity. can u help me,what do i need to do?

when i use console cheats my computer begins to slow,does the console take any of my ram free space?
I dont use console because of this,please can u answer me?

there are some codes, like npc_create npc_zombie, that i use but it says "unknown command: ncp_create" and i dont know what to do! help me please

the only portal game that i know is portal flash version. how do you get to portal orignal??? please i really want help please!!!