That would sort of defeat the purpose of having mini-itx. If you need decent graphics then a 780g solution would be the best IMO. You don't loose much in the way of size and the hd3200 igp is the best igp around.
W/C I could see, but I don't think the mini-itx boards support quads for one, second the gtx295 will be a major engineering challenge. Set top boxes don't have a lot of height to them. So I don't know how you would fit the card.
Philbar you could probaby fit a gtx 295 in a an old vcr or your 9800gtx and get a new one for your main pc. My idea would be to hide an optical bay behind where the video door is (taken of its hinges), turn it into a stealth drive so you push it and the cd bay comes out. Also do a horizontal (left to right) airflow if you can with possible drill hole fan grills so that the front and top seem stock.