It will fit the GTX285. Although wheter a 470w athena psu will be enough is a whole nother story and it's not like there is a better option.
it would be more than enough, did u try the gtx 260 with the stock psu? U know if its a 55nm or a 65nm card? im runnig HD4870 on stock psu with no probs at all, psu doesnt even get warm.
ive had my 800W atx psu in there and no problems, exept for the dvd drive.
in theory with a peak of 160w gtx 285 might work on stock psu
http://www.xbitlabs.com/articles/video/display/evga-geforce-gtx285ssc_5.html#sect0