Since many people wrote that overclocking an WNR3500L or RT-N16 will work flawlessly on max. clock frequency i decided to try it. (Since USB transfers from harddrive are really slow even with ext3)
I used the data from http://www.dd-wrt.com/wiki/index.php/Overclocking_BCM_47xx_CPU%27s to overclock to 532/266. This worked well for about a week with increased transfer speed from about 6MB/s to 7MB/s. (not huge but still noticeable) But than it seemed that the USB connection will stop working when the cpu load is high. It happens when downloading files with transmission or simply transferring files from harddrive over ethernet. There is nothing to see in the log. The USB devices just vanish from "USB and NAS" and after reboot they are there again. Unplugging and replugging doesn't seem to work either.
Now i clocked back to 480/240 and everything works again. Just wanted to share my experience. :) Does anyone else notice something like this?