You're asking almost impossible, the total would be near 570$ or so just for video,ram,cpu,mobo leaving aside the psu and case, try telling your friend to downgrade the video card and maybe then maybe it'll fit
800 dollars can get you a decent machine... you shouldn't skimp out on the mobo like that so i'd recommend uping your budget or not getting an 8800gts
hmmm im just more of an AMD person .............. sorry no offense :-/