An x-pen is just a folding exercise pen. Here are some examples of some:
Exercise Pen - Dog Supplies: Find, Compare, Read Reviews & Buy Online @ Yahoo! Shopping
They come in different heights, so what height you get really depends on if your dog is a jumper or not. A playpen will work if your female is not a jumper. It is just a bit more crowded. I like to set up the x-pen with the whelping box it it so mom has more room to get out and use her pee pads.
I do pray your momma has a safe whelping. Be sure to keep us posted.