It's really a personal choice, as far as whether to frame the sketch
with the cel or seperately.  What I like to do is go to a nice frame 
shop pretty early in the morning on a day off, for a better chance to get
personal attention, and I put the cel and the sketch in different
arrangements, hold it up to the different colors of mats and frames, etc
to make sure it's going to look just perfect!  At a good frame shop
they will want to look at different options and make it look even
more beautiful than it already does!  ^_^

If price is a major consideration, getting the sketch framed behind
the cel rather than beside it, above it, or below it, will be cheaper
because you won't have to pay for as much glass or mat or frame... but
do make sure they're not touching or it'll get stuck!  If money isn't as
important as having it look good, I personally prefer getting the
cel and sketch in the same frame, but seperated by about an inch of mat,
either beside each other or one above the other... but the most important
consideration should be what you think looks the best!  

One more thing--welcome to the world of cel collecting!  ^_^  Its really
expensive, but the cost is definitely outweighed by the fun you'll have,
the people you'll meet, and most importantly--the cels!!!  ^_^

Tifa