Going fanless with your case might be a bad choice. The requirement for fanless is convection, so side with the big fan which normal sucks in air should be facing up to allow heat to rise and escape. During load, your PSU would be getting hot GPU/case air forced throw it, which is fine only if you have dedicated fan or really good air flow setup.
As for silent operation during web browsing,... im very happy with my Corsair SF450
. Have had since May of last year. Never seen the fan every come on during normal usage. I've pushed as high as around 200watts for extended periods of time w/o fan coming on. But i also have mine in good fanless setup, top of case, vent out of top...
In short, if you want good fanless operation, you need to have it setup in a proper fanless setup. If you have good airflow, you'll probably be OK for idle/browsing usage. Remember fanless, means vent up for convection. Read the manual for the PSU you are looking at for the fine print, for fanless operation, it likely has quite strict requirements.