I want to upgrade the ram I use on my ASUS P5B MB. Its limit is supposed to be PC 6400 / 800 MHz RAM.  
 
Then I found this site: http://event.asus.com/mb/fsb1333/. I upgraded my bios to the version described here. Does that mean that I can buy PC 8500 RAM for this motherboard now?

I believe it just allows you to overclock your ram. Im not 100% though.

Ah...okey. So the max limit is still only PC 6400 RAM?

You can use up to PC2-5300 DDR2-666 after flashing the Bios.
You can put in faster memory, but the maximum speed will be DDR2-666.

But it comes out of the box with Dual-channel DDR2 800/667/533 compatibility. Would it not be higher when I flashed the bios?
 
http://www.asus.com.tw/products4.a [...] odelmenu=1

I am 100% sure that PC2 8500 ram will work the P5B just fine like mine in my sig. Can you tell me what version it is? All DDR2 1066 is is overclocked DDR2 800, when you get it up and running have a look at CPU-Z it'll show PC2 6400 even though is 1066.

Read the link:  That Bios flash is there to add support for the newer 1333 FSB processors.  If you have a 1066 FSB processor it'll still run on 1066 timings. Unless, of course, you tell it otherwise.    
 
Regarding your RAM:  You are nearly always able to insert faster RAM into your system and run it, provided it is of the correct type.  It will simply run at the default settings your Mobo/Bios have.  In the case of PC2-8500, you'll normally boot to an 800Mhz default.  Then you would have to go into the Bios, tell the computer to run the memory at 1066 (533Mhz apiece x 2 = 1066), and see...  Be a bit cautious:  If you have an overclock going already it'll drive your RAM that much harder as well.