good question.
as for the conduit question....: either way is ok code wise, conduit, or romex wire......pretty much anywhere in the world.......
as for the practicality of conduit,......its not really practical, unless either of these two things are present...: first, if your wire run will be on the floor of the attic, and is at risk of damge due to people walking on it, it should be protected in conduit, or if u have a rodent problem. if there in pvc pipe, they cant be damaged under foot, or by something chewing on them.....
i would say 99.9% of the time, just plain old yellow 12 awg romex from homedepot is just fine. dont pound your staples in too tight tho, if the wire is squeazed by the staple, heat can build up.
another example of a good use of pipe is, if you plan on running multiple circuits, which i doubt you are. but, if you were, u can pull several wires thru a pipe, rather then having to run 3 romexes for 3 20 amp circuits.....
may i ask what you powering??????
whether it be lights or fans, one, 20amp circuit should be fine.
romex wire, make for an easier "do it your self" project, as trying to run pvc, and fishing the pipe down your walls, to a box, would be VERY labor intensive.
i have staple romex to the floor joists of an attic a million times. its quite safe. you dont even need to staple it if their is no traffic except you up their, id just leave it laying their. pipe in attics can also, cause heat to build up. attics are very hot in the summer.
as for the arc fault vs. gfci question:
its best to use gfci. especially if your main concern in fire or electricution. arc fault is a new concept and hasnt been around long, but gfci protection, is undesputable. you really dont need either, unless water or a sink is near by, but, gfci recepticles, are 10 bucks, and the breakers are 100 bucks, id just go with the recepticles.
i can help you install these devices, if you need it.
only buy (1) gfci, per circuit, meaning, if you decide to go the cheapest easiest route, youll buy enough #12 romex wire, forget the staples, use plastic boxes, and standard devices, except the first device will be a gfci recepticle, with a reset button on the front of it. the one device, if used first in the circuit, will protect the entire circuit down the line. so only buy one gfci per 20 amp circuit.
dont hesitate to ask more questions on this subject, as i understand my info may be confusing. im here to help.