From DD-WRT Wiki
 The WHR-HP-G300N
- Wireless N
- 4 x 100mb/s ports
- Cable Router
- Atheros AR7240 rev 1.2 CPU (400MHz)
- 32MB Memory
Since the second half of 2010 the WHR-HP-G300N ships with a Buffalo supplied DD-WRT firmware installed by default. Older versions of the router are the same specifications and allow the DD-WRT firmware to be installed from the buffalo website/FTP server.
Buffalo refer to the DD-WRT firmware as the "Pro" firmware and the original firmware as the "User Friendly" firmware. Not to be confused but the "Pro" DD-WRT firmware is much more pleasing on the eye than the "User Friendly" firmware if not easier to use!
 Initial Installation
Buffalo now provide an official DD-WRT build direct from their own FTP site.
Acoring to Buffalo documentation the new DD-WRT firmware is installed by default on the WHR-HP-G300N however mine had the original firmware
The DD-WRT firmware is named as the "Pro" firmware and is installed by default. A "User Friendly" firmware is included as optional.
To download the firmware grab the zip file from ftp://whr-hp-g300n:firstname.lastname@example.org/
Extract the firmware from the zip and use the web based firmware upgrade interface as usual to upgrade to the "Pro" DD-WRT firmware from the stock "User Friendly" firmware
 Firmware Versions
To find the latest community DD-WRT builds for this unit you would need to browse the FTP and look for the appropriate build for your unit. Below is the link for the file server.
To find buffalo DD-WRT builds you would need to go to buffalo's support site and find the downloads section for your unit.