Sounds like it could be a hardware problem, either your PSU or your Graphics card.
Its hard to tell exactly, you should try putting your 8500GT back in and seeing if the PSU is still getting really hot and if your performance goes back up. If your PSU is fine and your performance does go up, id say return your gfx.
Also does your case have good airflow? As the PSU and the GFX could be making alot of heat, and if they are close together... spells trouble.
Also, it could possible be bottlenecking, but not enough to actually reduce the performance of the GTX 285 to lower then a 8500GT!