Can anyone tell me what an acceptable server load is? Is a server load of 0.98 too high?

Thanks.

0.98 is just fine. I think the serverload is considered when it goes 2.00.
I am not guru at these so you might want answers from gurus.

Many thanks. Do you know what happens when the server load gets too high? Does this hust result in slow HTTP access?

Well, you can assume.
I would certainly be sure that HTTP/FTP/Mail server would slow down. It will also cause the heat if it's overloading for a period of time.

When the server starts crunching the other processes (apache, mysql, etc) become slower.

This is because unix/linux works like a circle. If there is only two processes the CPU takes turns giving each processes cpu time.

Well if there is some other things that need CPU time, then the other processes suffer.. then you can get technical and start talking about process priority, etc...

but typically try to keep it below 1.00