At 40%, the fans aren't terrible. Any higher though, and they get exponentially more audible.

As for the PSU question, it looks to me from pictures that the e520 uses a proprietary PSU, just by looking at the screw placement. You'll have to look for a PSU that specifically says it will work in that system. You should check your PSU to see what wattage it is...my guess is that it is in the 300W ballpark, in which casethe best you can do is a 9600GT without a new PSU.