Those settings are to set the speed of your memory in relation to the speed of your FSB (CPU).
CPU speed : Memory speed
1:1
133mhz / 1 = 133 ....133 x 1 = 133mhz memory speed
100mhz / 1 = 100....100 x 1 = 100mhz memory speed
3:4
133mhz / 3 = 44.3....44.3 x 4 = 177mhz memory speed
100mhz / 3 = 33.3....33.3 x 4 = 133mhz memory speed
4:5
133mhz / 4 = 33.25....33.25 x 5 = 166mhz memory speed
100mhz / 4 = 25....25 x 5 = 125mhz memory speed
Get it? These ratios should also apply during OCing as well...just plug in the CPU FSB and work with the applicable ratio being used to determine what your final memory speed is going to be. Ratios are handy for determining the Max CPU OC as you can relax the memory using the 1:1 ratio. Then if your memory is up to it...apply 3:4 or 4:5.
The P4PE you are considering I believe also has AGP/PCI lock...able to hold these busses at 66/33mhz no matter the FSB...most desirable for OCing.
These CPU:Mem ratios are getting more common amongst P4 DDR. Boards with SiS chipsets tend to have more ratios available...MSI648Max for example: