Its all good saying you will get a new card but you need to check that your psu can handle it, make sure you have enough amps on the +12v rail(s), does the case have good airflow etc.....? The last thing you want are heat problems.
Yes the PSU is important, first thing to check is if he even got 6(6+2) pin connectors for PCI-E cards.
Judging from what he is saying that he got a big CPU cooler and a chassi with LEDS ill guess it's a "gaming" chassi with decent airflow.
Tho from experience both the 6850 and 7770 can run in nearly any case if you are using ref coolers that blows out the air in the back brackets. Any midi-tower case with two fan places will be sufficient enough to keep them cool and since he have a big CPU cooler he will be fine, nothing to worry about.
Ive ran a Radeon HD 7970 in a case with one front 120mm fan and one back 120mm fan + PSU outtake without any troubles in stock while I was waiting for my Cooler Master Cosmos II case and that's AMD's fastest single GPU card, sure it ran a bit hot but well under the limit.
EDIT: Saw that Lassie posted before I was done
He has build it himself the man i purchased it from, with the biggest Air cooler he could get, there is 4 in it, and they are huge, he said i never had to worried about it will run hot. As for the power he said it could handle a lot. I am not a expert So how can i check amps on it?
Check the PSU, it has a sticker showing the amps on the rails, or just simply write the PSU name and model and ill check it for you
Any decent 400-450W will have enough AMPs for both cards I wrote as long as they are not cheepo PSUs.
One of my Folding@home machines are running a GTX 670 at full load 24/7 on a 450W PSU that has 38A on the 12v rail, people tend to read the total W and it seems people allways buy a lot bigger PSUs then they need.