First off, welcome to the forum.
Now, for the record, you do not need to modify the pin header in the case, depending on the motherboard you are using. Only if you are planning to get an ASUS motherboard, you will need to do this.
Next, I remember from my XPS 730x, and that cable you ar referring to is not the correct cable for the front panel and power button anyway. That cable leads from the front buttons to the MIO board. You need to find the cable labeled TY407 (i think) and that is the cable used to connect the motherboard to the front button. It should be a 10 pin cable (5 x 2 pins) with one blocked off.
One thing you must understand, is that on the XPS Gaming series and even now with the Alienware's, almost everything is run through the MIO board. And the cable you are referring to runs from the front of the chassis, connects through the MIO board, and then there is a second cable, which is the TY407 (i think), which connects to the motherboard.
Now as for knowing which pin is which in case you do get an ASUS board, the pins are the exact same as the XPS 630i.
Have a look here
and check out the section with the ASUS Mod links, and it will explain everything you need to know about it.
BTW, just for curiosity sake, which motherboard were you planning on swapping to? The XPS 730 already comes with a 790i motherboard, which is practically the best LGA 775 motherboard when it comes to overclocking and functionality. Are you planning to go to an i5/i7 setup? Keep in mind also, if you have the H2C cooler, the northbridge cooling block on an H2C unit will only mount to the Dell motherboard, and pretty much the same with the CPU block, as it is measured in length and height to fit it.
And you should make a sig with all your PC specs as well, for the future with helping out.