How Hot is too Hot?

In general it's a little bit too hot, but nothing extreme.
The more important question is what is it getting up to under load?

For air cooling - I generally aim for an idle temp. of around the low to mid 30's in degrees celcius - but the exact temp. depends on the CPU type, the heat sink, plus case cooling etc.

I've managed to get quite a few overclocked CPUs down to low to mid 30's on idle, using Air cooling, but my current CPU is a little higher than that - it generally sits around 40 to 42 at idle and gets up to around 60 under load, a little more if the room gets warm, which depends on what the weather's like outside etc.

So in a nutshell - slightly too hot, but I wouldn't worry about it if your load temp is fine.

You might be able to drop it a couple of degrees by altering your case's air flow and ensuring cables inside the case are tucked out of the way etc. Thermal grease is another easy way to get back a degree or 2 as high quality thermal compounds are more efficient at transferring the heat to the heat sink etc (eg: Artic Silver 5's a good all around thermal grease). Consider using a good after market cooler on your CPU if you're not already...

Thats a little hot for that CPU, I have a 6400+ BE using a Thearmaltake TMG A3 heatsink & fan on one system that idles at 40c. The 6400+ BE is a 125 watt CPU. It hits 57c under load and this fan heatsink combo is just a real cheap one.