Best leave it at default, especially on Vista. It will resize the file if it needs and the overwhelming majority of times you modify the swapfile from default, you lose performance.
You CAN put the swap on a seperate drive, if you want, but make sure it's the fastest drive in the system. Putting it on a seperate partition on the system drive won't do you any good -- that simply moves the swapfile towards the end of the drive and forces the heads to jump around more than necessary. You shouldn't be worried about defragmentation or anything like that.
Fiddling with swapfiles is so 90s.