Graphics Ideal Temperatures for a X800 XT?

I've got an Asus X800 XT that I bought at Christmas and added an Arctic Cooler to it. For reference, here's the rest of my PC specs:

Asus P4P800-E motherboard
P4 3.2ghz 800fsb
1gb DDR 400 RAM
200gb Maxtor IDE ATA133 hard drive
450w PSU


Now yesterday was a pretty warm day and after a couple of hours of UT2004 I switched to Doom 3: Resurrection of Evil. Everything was fine until it stuttered going through a door. I thought it was just a fluke, only lasted a second, maybe someone signing in on MSN as i forgot to switch that off and i usually makes games flicker or something. But in the next room there was an x-ray machine (like the one in Total Recall if you've seen that) and the game slowed right down as soon as I was in the room with it. If I looked away, it ran smoothly, looking at the x-ray machine made it grind to a halt.

My PC should be able to handle it - the only thing I could think of was that it was overheating so this evening I've installed ATiTool, but I've realised that measuring the temps means nothing unless I have a rough idea of what they should be.

At the moment its quite warm, according to ATiTool the GPU temp is 44C when idle, it ran at a steady 75-ish C when playing UT2004 for about 45mins. Not sure if this is good or not, anyone know what it should be ideally running at?
Also, I was playing around with the settings in ATiTool and it seems the fan only runs at about 30% by default. I've changed the settings so that it increases with temperature, and that's what it was doing when i played UT, so it might be a bit hotter than that without the ATiTool fan settings.

Any help/advice appreciated.