The GTx 660 is within your PSU range so you wouldnt have to replace the PSU but im no expert i recommend doing abit of research or reading what people have to say hope this helps
PS if your willing to buy a new PSU you will beable to buy a much better GPU the R9 270x seems to be a decent card but you will need over a 600watt psu to support it i recommend also that you list your budget or you will have people saying buy the PNY Quadro K6000 12GB which is £4000