hmmm considering a HD6970's max power draw is 300watts(200watts peak power draw). 750watts would be enough for two cards IF he doesnt overclock.
http://www.techpowerup.com/reviews/HIS/Radeon_HD_6970/27.htmledit: true its abit of a thin margin

@ OP
Pros of building your own:
you get to pick your stuff
more options to pick from
you know what parts you're getting
warranty on each individual part
the fun of building it
boast it around!
Cons of building your own:
abit of a hassle to build it
less discounts
you need to research first
troublesome for the beginners
Pros of buying a prebuilt:
discounts
almost hassle free
boast that you bought a prebuilt?
Cons of buying a prebuilt:
you dont know what parts they used
limited choices
limited upgrade path
limited warranty