gauge lights and header Options

[my96celica]

my gauge lights stop working so i had to buy a light to see at night and i was woundering what it could be?I also have a header on the car so after i put it on i reset the ecu because i was told here that it would help the hp gain is this true?does anyone know how much hp u really gain from a CAI and header?I was told the header does not give any hp gain.also when i let off the clutch to fast my car gos knock knock knock and i was told it could be a motor mount i am thinking its an axle but it cant be the ball joint cause i had one break on my integra b4 and it doesnt sound like that.does a battery voltage stabilizer and ground wires really increase hp?by about how much?is it worth it?ill get it even if it only makes the car run better instead of being faster.ty for the help