Distinguish between physical disks and partitions - there is no great benefit from putting the swap file on a different partition on the same physical disk - the disk heads are working just as hard!
It can be useful to move the swap file from c partition to d partition if C is getting full. Normally one would have a smaller C (system) partition than D (data), so C would tend to fill faster.
It only makes a difference when you can put the swap file on a physically separate disk. But as to using an old disk - not necessarily a good idea, unless the seek time and DMA support is adequate.
If you have a 5400 rpm (or faster) disk with UltraDMA mode 4 or 5, fair enough - but it would probably be actually a retrograde step to introduce a slower disk without UltraDMA mode 4.
SD