August 24, 2011 2:51 PM updated: August 24, 2011 5:26 PM

This is what I have read somewhere in the past, it might have changed.

The usual of only 1 type of Wifi so only B or only G or only N.

Then select the spacing 40mhz is for good signal and few if any wifi/portable land line/microwave/other around you that could interfere with the signal.
20mhz would give you more distance and possibly better performance.

Wifi channels 1 is where all and everything always wants to end up using, try higher numbers closer to 6 to 9 this is usually good with USA junk in the house electronics and their interference.

Look for the channel when you are in a weak signal area that is the most responsive
ie: via lan cable your dd-wrt status page opens in 0.3 sec or whatever is the fastest and then via wifi does it come close or same or better = this should then be a good wifi channel. Measure with each channel until you hit optimal.

I found a sweet spot at ch7 for me.

Once this is done then usually in dd-wrt you would do a site survey and refresh that page as many times as possible and try to keep track of what wifi router names stays and what drops and on what channels they are.

The ones that is always there would be your greatest competition for wifi channel.

Wifi broadcast strength, if you can get away with the default 17mw then good but with normal interference try closer to 60mw or 81mw or 128mw depending on your firmware. Try the weakest mw setting that gives the best performance.

Not sure if the "make permanent procedure" is fixed if not then try save + apply + reboot and then test new power setting. Again depends on the firmware number version.

SNR a good value should be 20 or more for the weakest device listed.

Also do a test with a tomato firmware and use the "device list" to make sure you have both 18/18 minm. for any devices, dd-wrt does not have this nice feature.
Aim for at least 54/54 if possible or more with the power settings.

Then test again. Also note that the USB driver EHCI could be unhappy so usb testing will end up at this slow speeds where pc to pc should be much better.
Also test 1 lan pc to 1 wifi pc and or get visualware.com server software and do full proper testing on both lan to wifi and reverse with this program.

PS> you can also use one of the many free android apps for wifi scanning like metageek insider, you have to go to their website to get a link to android market to get this free app. and then test on your android phone as well.

Hope this helps.

EDIT:
I can achieve 5.8MB/s on usb stick class6 and usb dual rotational 7200rpm drive enclosure gets to 5.0MB/s pulling from RT-N16 to pc via lan Cable with dd-wrt-usb-ftp-samba3-dlna-O2-v24-K26-broadcom-14.bin