Zack, Bobby is correct on the rental issue-only aramark as they have the concession contract for the lake. There are diferent marinas to rent from though and I see you are in Texas so you will probably go out of Hite or Halls crossing at the more eastery end of the lake, I have little experience at that end but have heard some good things about it.
You will for sure want the Admiral class boat with 12-15 people. Obviously enough food, drinks, water, sunscreen, etc. for the week, as far as gas goes bring as many plastic fuel cans as you have room for, we bring 90 gal. in plastic drums and our houseboat has a 140 gal. toy tank and this works for 2 boats for the week.
here are some sites to check out. http://www.elakepowell.com/freds/index.htm http://www.summittech.com/LakePowell/LP_WaterDB.php http://www.wayneswords.com/