the only way you are going to really know the wattage of your psu is to open the case and look at the psu. you are going to need 450 to 500 watt psu to run the gtx 460. I am basing this of of this article.
http://www.guru3d.com/article/geforce-gtx-460-review/13
I figure it would be ok to upgrade to the 460 as i believe you have the 260 (just assuming) in your machine, and the 260 consumes more power.
Your 260 (assumption) might be fine it just might need a bit of cleaning, maybe it has to much dust in it. however even the 460, will bottleneck your cpu so an upgrade will improve the performance of your system, and thus more eye candy.
I don't know to much about the case, so i cant comment on the thermal displacement efficiency, nor can i comment if you can upgrade case fans. Your going to have to do some digging.
Your case cooling is probably OK if your CPU has its stock heat sink and you have decent temperatures. You could also check your hard drive temps to check if those are in the normal range as well. Doing this will at least give you an idea if just the GPU is hot or all components in the system are hot, and thus your case is lacking in cooling capabilities.
Here is a good system monitor program HWmonitor, available at cnet, or good ole google.
http://download.cnet.com/HWMonitor/3000-2094_4-10793486.html