Hi
If the 2nd HD just has data on it and not an OS then its easy to do, just buy an external HD caddy with a drive big enough in it, most have a USB connector, plug it in and use explorer to copy the files to the new drive.
If however the 2nd HD has an operating system on it, then you will need to use a program such as Norton Ghost or Arcronis True Image to copy the drive, this is easier to do if the drive is connected to an IDE cable rather than on an external drive as it can take an age to copy an entire 20GB drive across USB.
Most external drive caddy's take the small 2.5" laptop drive as you plan to put the copied drive back in you will need to get an external cage for a 3.5" drive, these require a seperate power supply.
But I would do away with the external drive Idea and just fit the new 3.5" drive into the PC, you can connect it to the CDRom IDE cable as a temp measure whilst you copy the data over.
DOnt forget that once you remove the old drive make sure the jumpers are set correctly.
I think that covers it...
Regards