Man this thread is so far back.

I recently did my first repeater setup, using an Atheros based router, something that I regret .

Is it possible to repeat a repeater? As in the second repeater is not connected to the AP, but to the first repeater (which is connected to the AP). If yes, should the gateway (of the second repeater) be the address of the first repeater? According to the DDWRT forums this is possible, but I could not find a clear answer on how the setup should be.

Another thing is DHCP doesn't seem to work for wireless, I'd have to use static ip to surf the net. It appears that many people using Atheros are having this problem.

40Mhz is for wifi N 300Mbit/s.

Yup... plus some routers work better with the stock firmware. One example is TP LINK 1043ND. I hope the developers will make a ddwrt special edition for it.
Currently it's the most voted atheros based router to get the special edition.