From DD-WRT Wiki
 Hardware Information
|CPU Speed||680 MHz|
|Flash ROM||16 MB|
|Radio||1x 2.4GHz and 1x 5GHz|
|WLAN Max Speed||300 mbps|
|Switch||4x GigE LAN + 1 GigE WAN|
 Flashing Instructions
If you have an WNDR3800 with stock firmware, and the version is greater than 184.108.40.206, then you will first need to downgrade to firmware no greater than 220.127.116.11 (from here). Netgear put a check in firmware greater than 18.104.22.168 to allow only Netgear firmware from the Web GUI. It hasn't been confirmed if you can flash DD_WRT using TFTP with stock firmware greater than 22.214.171.124.
You will need these files for the flashing procedure.
If you have a North American model, you will need this for the initial flash.
If you have an EU or Asia model, you will need this for the initial flash.
The other file located within the WNDR3800's dd-wrt dir is for upgrading to newer or older dd-wrt builds from within the dd-wrt webgui. Its naming scheme looks like so - wndr3800-webflash.bin
For going from 126.96.36.199 Netgear firmware to DD-WRT on a WNDR3800, installation goes like this:
- Set a static IP such as: 192.168.1.3 subnet:255.255.255.0 gateway:192.168.1.1 on the PC you will be using for the flash.
- Connect the PC to one of the router's LAN ports (unplug your ISP modem for the moment)
- Make sure to Reset to Defaults on the router prior to flashing. This can be done by performing a Hard Reset
- In your web browser, go to http://192.168.1.1
- Find the “Router Upgrade” in the menu and enter that section.
- Upload the .img firmware file, that you downloaded from the link above.
- Wait 5 minutes after the router says it is done.Make sure you clear your browsers cache between updating firmwares.
- Perform one more Hard Reset, then login at 192.168.1.1 and begin configuring.
- Enjoy dd-wrt and do not forget to donate if possible.
- Should you want to use DHCP ip assignment, remember to un-set the static ip that you set in the above instructions.
This unit was supported as of build 18687.
For updating to a newer dd-wrt build, you should be able to use the wndr3800-webflash.bin mentioned above. Make sure to reset to defaults with a hard reset as listed above and download the appropriate dd-wrt build that you are wanting to update to. New builds can be found within the dd-wrt file server: