fan speeds

I have two fans with heatsinks what I want to know is.
1 does having a higher rated fan speed ie 6000rpm help to keep cpu cooler or is it the size of fan 80x80x25mm.?

There are many factors that contribute to the CFM rating on a fan such as the # of blades, blade design length / width / pitch, RPM, size of the dead zone ect. But a typical rule of thumb is the taller and or larger diameter the fan is the lower the RPM needed to achieve the same CFM

how is it then that when i change from a sibak cooler to the original fan and heatsink that came with my xp 2100 it shows 5037 rpm and a cpu temp of 0c/32f but when running sibak it shows 40c/110@2500rpm

thanks.

That sounds like your motherboard temp sensor is messed up, (unless your in the Arctic) 0C is freezing and with any air sink, at best you will still be a bit above room temperature (ambient)
if it changes every time you switch fans try using a molex connector instead of the MOBO fan headers and see if that cleans it up a bit

where can i get a molex connector