ISO800 doesn't result in more noise, Yousuf... it's metadata...
You are shooting 800 to protect the highlights. Think of the "native" sensitivity as 320... shooting 800 with a right bias on the histogram allows you to expose the sensor well and essentially makes you more careful of the highlights because your view of the image is showing more light sensitivity.
I really don't know how better to explain it - you just need to go shoot. Expose your shot nicely with 800 ISO then switch to RAW view to see exactly what the sensor is seeing... the image is pulled down a bit but still looks great.
800 is your view
320 is what the sensor is "seeing"
Hope that helps.