According to the NVidia site http://www.nvidia.com/object/product_geforce_gtx_285_us.html the 285 requires a 550w psu minimum. I could not find the amp requirements. Wattage-wise your psu sneaks by, however from my research your psu has one 6-pin connector (the 285 requires 2). It is my understanding that you can use 2 molex connectors with an adapter to get the power for the 6-pin though. I have heard both sides of the fence in regards to power for graphics cards. One side saying card manufacturers grossly overestimate power requirements for ensured stability and to take into account systems with extreme hardware that most don't run (highly OC'ed cpu, raid hdd's, etc.). The second side saying you should always be on the safe side, because an overtaxed power supply can lead to damage of attached hardware. This site I personally have referred to many times to check for power requirements. It will probably help you decide. http://extreme.outervision.com/psucalculatorlite.jsp
Hopefully this helps.
Your psu should have a maw output rating listed on it. I did a little more research and I think that psu has a rating of 34A max. From what you found the required amps are 40A for the 285. This makes me think you may not want to try it. That's my opinion though. There are a few guys that really know their stuff about psu's in these forums. Hopefully, they will chime in. They certainlu would be more of an authority on the subject than me. Not that I'm an idiot, I just don't want to steer you wrong one way or another.
I wouldn't use it. The 285GTX pulls about 183w at load that is a little over 15A. Since your +12v rail(s) is shared with other components they are recommending 40A or 480w available for the 12v+ rail(s). That PSU only has 34A (408w) on the two 12v+ rails and 460w available from that PSU at full load. http://s164.photobucket.com/albums/u12/jdewinnaar/?acti... It is not a very efficient PSU and really a 460w PSU not a 585. I think you may have problems especially if you OC.
A better quality PSU may have about the same or less watts but make better power. ie. Corsair 550w or PCP&P 610 Silencer that make 41w/49w respectively for the +12v rail.
EDIT: Even a 260GTX at 36A would require more than your PSU has available. You could drop back to a 9800GTX+ as that only requires 24A on the +12v rail and still is a great gaming card for most titles at 1680x1050 and lower resolutions.
Thanks for the replies
ummm... Im starting wonder about the psu.
Will the 720w version be a better bet?
We got the 285 cause they where out of stock of the 9800+ which im happy with. The 285 is a more powerful card. We got it at stock price witch is even better, we got around 15% off to a normal retail price.
The Gigabyte 720w has 42A available to the three 18A +12v rails so it would work fine. It is expensive compared to the two I mentioned in my last post as well as several other choices available with 40A or more. Do you just like Gigabyte, getting a great deal, or only have limited choices where you live?