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Here is a link to a review of the HX620 with a preview of how loud it is.
http://www.youtube.com/watch?v=4TrEHFpP7Y4
and the same thing with a Silverstone ST75F. I have a ST70F and i can tell you that everthing else in my case is louder than the PSU. Very quiet...
Do you NEED 16GB of RAM availability? You say your perfectly content with the board your using, so unless your just looking for something to do, or don't mind blowing money then by all means, SPEND SPEND SPEND.
But you sound satisfied with your current setup so why upgrade?
Don't get me...
On another note. 60C should be fine for that card. My old 8600gt idled at 60C and i never had problems with it even under load. And i ran that card for about a year 24/7