I am going to be updating my HP P6204y with a new power supply (Corsair or Silverstone) and video card (likely an 460 GTX) and since the motherboard is upside down in this build, the CPU is near the bottom of the case with the case's only dedicated exhaust fan in the same general area. My new graphics card is going to be fan-side up while the new power supply is going to be drawing air from the same area of the case (since the 250w power supply I am replacing drew air from its front and pushes it out the rear, this is naturally going to cause a change in airflow).
The two components would be about 4 inches apart.
The more open side of the case allows airflow in a few areas, and I've enclosed a picture of what that side looks like.
I think you should be fine on heat, as the PSU will probably run more efficient than the OEM and the GTX 460 was a decent card for heat. I would recommend monitoring your temps closely until your are confident they are safe. Might want to run Prime95 for an hour or two to see how your temps are under a full load. I recommend HWMonitor to monitor your temps with.