dude, why ask a question and then argue about the answer.
if youre getting a 7900GT video card, youre going to need a really good PSU with lots of W.
try a Fortron 400W (thats what i have, but i dont have a 7900GT) or even higher. a 250W PSU or a 300W is NOT going to power your video card enough for it to work like a 7900GT should.
if you want this tiny pc so bad, why not just downgrade your video card to something like a ATI Radeon 9800 pro (lol). that will work jusssttt fineee.
(ati radeon 9800 pro SUCKS).
seriously, if youre getting such a high-end video card, you need a great PSU to go with it. get that antec 550W that was mentioned. thats supposed to be great.
P.S. try working on your grammar, you keep missing words and its hard to understand what youre saying.
P.P.S. i apolagize if english is not your first language, as that can not be helped.