From DD-WRT Wiki
The orig firmware is based on openwrt.
Since Netgear changes HW between Rev. depending on the cheapest OEM, the Rev. v3 consists of athe following HW:
- CPU: Atheros AR7241
- RAM: 32 MB
- Flash: 4 MB
- LAN/WAN: (4+1) 100 MBit/s
It only allows to flash builds of max size 3648 KB. So that's why sometimes the builds wont flash, they are simply too big.
 Initial flashing instruction
For initial flashing just flash the appropriate DD-WRT overtake image file (WW (Worldwide), NA (North America)) though the web interface. After the reboot DD-WRT will answer at 192.168.1.1. Future upgrades are done though the GUI to the latest builds.
As of 20140119 you can only install DD-WRT if your Netgear fw version is < 188.8.131.52 (ticket). Netgear decided to limit the orif fw size, so you will have to downgrade orig firmware to be able to flash to DD-WRT.
Steps for flashing:
- Read this thread. All of it: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=172030
- Download this file. You can download without registering: http://downloadcenter.netgear.com/en/Disclaimer.aspx?redirecturl=http://netgeardownload.registria.com/single_page_registration?product[sku]=wnr2000v3&download_url=http://www.downloads.netgear.com/files/GDC/WNR2000V3/WNR2000v3%20Initial%20Release%20Firmware%20Version%184.108.40.206.zip
- Unzip the file you just downloaded.
- Read this wiki article and download the tftp.exe program (for windows only) http://www.dd-wrt.com/wiki/index.php/TFTP_flash
- Download the 18777 dd-wrt flashing file from here. Choose either the NA (North American) or WW(World Wide) depending on your location. ftp://ftp.dd-wrt.com/others/eko/BrainSlayer-V24-preSP2/2012/03-19-12-r18777/netgear-wnr2000v3/
- Also, from the same location, download the webflash.bin file.
- Attach your computer to the router with an Ethernet cable, and disable your computers wireless.
- Do a factory reset on the router. Test this by logging into the router at 192.168.1.1. You should get a logon page that requires you to enter "admin" as the user name and "password" as the password.
- Turn off your router and turn it on again while holding the reset button in. Hold it in till the power light blinks green. It will flash yellow first, then it will flash green. You can release the reset button. Note that the reset button is a LONG way in. You will not reach it with a pen tip. Use a paper clip.
- The router's DHCP server has been stopped, so you will have to set a static IP of 192.168.1.9 on your computer in order to connect to the router. If you don't know how to do this, pack everything up as you should not be messing with third party firmware. Quit the process, and come back when you understand basic networking.
- Start the tftp.exe program and set 192.168.1.1 as the server, leave password blank, and for the file, browse the file directory and select the netgear firmware you downloaded. MAKE SURE THE FILE ENDS IN V1.01.26.img!
- WAIT for the file to upgrade and the lights on the router to ALL stop flashing. This will take about five to ten minutes, and if you don't wait long enough, you can brick your router. The lan port will blink for a while, then the power will go yellow, then the blue wireless light will come on, then the power will turn green. At this point you should be able to continue.
- Log onto the router at 192.168.1.1 and you should see that the webgui is different and there is no version number anymore.
- Go to the Router Upgrade tab (under maintenance) and chose to upgrade the FACTORY_NA(WW).img (CHECK TO MAKE SURE YOU ARE FLASHING THE RIGHT FIRMWARE THAT HAS wnr2000v3-factory in the NAME!!!) and then WAIT again.
- When the DD-WRT login screen appears, you have to TYPE a username and password into the boxes displayed.
- Go to administration/firmware upgrade and change the first box to reset to default after upgrade, then choose the wnr2000v3-webflash.bin file, then hit upgrade.
- WAIT again, until you again get the dd-wrt login screen when you browse to 192.168.1.1, retype a username and password, and then you can configure your router.
- Don't forget to set your computer back to auto IP and auto dns.
- DO NOT UPGRADE TO A NEWER BUILD AS NEWER BUILDS ARE TOO BIG AND WILL BRICK YOUR ROUTER!
 Update 2016/07/19
Successfully updated to 29837 via webflash from a previous version. All more recent versions are over the 3648k file size limitation of the partitioned nvram. Multiple 30/30/30 resets caused unit to brick, but I recovered following above instructions, TFTPing Netgear stock firmware v220.127.116.11 and then upgrading to factory_NA image, then to 29837 again. 
If you have a bricked device or you want to go back to the original firmware you can flash the device using the recovery boot procedure.
- press and hold the reset button
- power cycle the device
- keep holding the button until the power led flashes
- start to transfer the firmware with an tftp client to 192.168.1.1
- when the transfer begins release the reset button
- wait until the unit reboots