iMac flickering and going to safe boot

My iMac 500gB (21.5 inch,mid 2010 ,iCore3, 3.06GhZ, 4GB 1333Mhz DDR3).

Question 01 :of late it has been flickering and/or going to safe boot.sometimes the screen goes steaddyand starts flickering if you click .What could it be,What can i do to rectify.(Went on you tube, was told press CTL,CMD,P,R .IT sometimes work but not always

Question 2 I intend to either upgrade my *GB and storage to 1 TB is that OK.?

Question 03 Can I trade it and get a new one at any Apple shop?