Whoa, the whole drive? How big is it? The page file only needs to be 1.5 to 2 times the size of your RAM. It might boost performance to put a "normal sized" paging file on there.
It would be a huge waste to use an entire 80GB HDD for a page file. You would get no performance advantage if you use it. Like m3trj said, use 1.5 times the amount of RAM in deciding the amount of Page file. It will however, give a slight performance increase if you put your page file on a separate HDD than your OS.
It's because hard drives are comparitively slow to the rest of the computer. When the hard drive has to constantly read and write from 2 places on one hard drive (from the OS and the Page file) it has a lot of work to do. On the other hand, when a whole different hard drive gets the task of reading and writing only 1 thing, then it is much more efficient.