Now, the wrt54G has always been 16/4 until the switch to VXworks w/ v5
The wrt54GS was 32/8 until it went 16/4 with v4.0 and 16/2 with v5 and VXworks.
If you want to buy a new, linux version, the wrt54GL still runs linux (that's the L) and is 16/4 like the wrt54G v4
So, the bad news is that they switched to VXworks and the RAM/Flash reduction outweighed the licensing costs. However, people were buying enough of them because they could modify the linux firmware to justify a continued linux version.
I have the 16/4 version and there's more than enough room for dd-wrt to work. Plus you can always mount a share if you need more space for logging or whatnot.
I love mine, but I think I'm do for a firmware upgrade.