Welcome to the forum, Briggs.
The conduit you use should be approved for electrical use. White PVC is for plumbing. You can find grey PVC that is made for electrical/ communications work.
"Low voltage boxes" is a little vague. If you are talking about something like
this , note that they cannot be used in fire rated walls.
Using cat6 for voice is completely pointless. If you want cat6 for data everywhere, then that's good, but cat3 is more than adequate for any voice needs. If it were my home, I'd run one cat3, one cat6, and one RG6 (voice, data, video) to most receptacles, in 3/4" ENT.
I'd have to look at the 08 NEC to find the section(s), but I'm fairly certain that you cannot run power and communications cabling through the same stud/joist/plate holes or conduit. I'd also surmise that using CMX rated cable is not acceptable when penetrating multiple floors.
If your attic is big enough to ever be a finished attic, you may want to consider running all cable down into the basement rather than in the attic, first floor wall placement permitting.
Installing a 3" vertical conduit in a home may be tricky - I'm guessing at some point it will run through one or more 2x4" top/bottom plates, which are 3.5" wide. A 3" conduit is 3.5" wide. If the walls through which it will pass are all 2x6 or larger, then you should be okay (you'd still need nailplates by code unless the conduit is EMT). A 3" conduit to service the v/d/v needs of just the second floor of a home seems like overkill to me.
All vertical penetrations as well as those through fire rated walls need to have appropriate firestopping.
This should go without saying, but all cable runs should be homerun from the jacks to the basement.
Jack
