http://www.dd-wrt.com/wiki/index.php/Supported_Devices

DD-WRT will work on v5+ WRT54Gs, just not as well. The v5 versions began the switch away from using Linux by default to using VxWorks on half the ram or half the flash or both, can't remember which at the moment, but it can still be flashed with a minimal build of Linux. However, there are cheap devices on the market today that have considerably better specs that'll work just fine. For instance, Best Buy, CompUSA (assuming they haven't been completely looted yet), and MicroCenter all had the Buffalo WHR-G54S in stock back in Feb.

-Sean