I've just finished building up my sub boxes and can't decide whether to carpet them or just paint.(paint's attractive because it's easy).
are there any benefits from carpet or is it purely aesthetics?

second question : what material do i use to line the inside of the box to absord the sound waves. foam?? etc??

cheers.

1) What ever takes your fancy. It wont affect the sound.

2) Don't use anything. I have never been able to hear the difference between a stuffed enclosure and a non stuffed.

1) Paint it and carpet it. Wood absourbs moisture, with paint it will seal it so it doesn't and last as long as possible. The carpet goes even further to protect it. I also means it will move and slip around less.
**keep in mind a box thats bolted down is a better box.

2) Dacron is the material many people use. As per SPL lancer's advice its a topic of discussion that some people see differently on. If you do want to try it, its available from many places including Jaycar and specialist car audio stores.