Well friend of mine did some tests, CPU is A64 3500+ Newcastle core:

a) 220x10 , 166 divider (183.33 MHz memory speed), using it as a reference value since 166 divider is "clean"

Sandra - 5518/5480 MB/s (integer/float, buffered,SSE2)
Sciencemark - 5079.09 MB/s

b) 200x11, 180 divider, (memory speed is either 183.33 MHz or 169.23 MHz)

Sandra - 5122/5114 MB/s (integer/float, buffered,SSE2)
Sciencemark - 4750.25 MB/s

Since bandwidth is noticeable lower, I think that memory is running at 169.23 MHz thus upper limit for memory clock for 180 divider iz 183.0 MHz rather than 183.33 MHz.

Still I'm waiting for one test - 220x10 with 150 divider which will give memory speed of 157.14 MHz, just to have another reference value to comare with.

Any suggestions?