NVRAM should definitely be a top consideration. If you want to add any sorts of third party tools or customization to your router you will want plenty of it.
I have 32MB NVRAM to play with and that has served me well enough, but I see plenty of consumer-routers which can be "upgraded to dd-wrt" but which doesn't even have enough NVRAM to do an ipkg update.
The short version is that, depending on your usage, you may run out of NVRAM before RAM and may lose your settings. For reference see [1] and [2].
[1] http://www.dd-wrt.com/wiki/index.php/Asus_RT-N16#Current_Kno... [2] http://www.dd-wrt.com/phpBB2/viewtopic.php?p=506126