In your stead, I would make it my goal to put the new drive in primary position, and the other one in secondary (or better, on the other IDE cable).
To do so, you will need to first hook up the new drive as slave and format it. Then I would try finding some software that can make an image copy of the drive. Norton Ghost comes to mind, maybe you can get an evaluation version.
If so, you can simply use Ghost and find yourself with two identical drives - one being 4 times larger than the other.
Then you can erase your current 20Gb drive and put it in slave, formatting it to use it as a basic data storage area.
On the other hand, if you have a 20Gb drive, it probably means your PC has already been used for a while - I'm thinking of how cluttered the registry of your Windows must be. This might be the ideal occasion to do a full, clean reinstall of Windows.
Doing so will not destroy your data, since you'll pull out the old disk and install on the new one. Once you have your new Windows OS up and running (with video, DirectX and sound drivers updated from the web), then you can pop in the old disk and copy your data files to the new disk.
All that will be then left is to format the old disk and use it as add-hoc storage.
Also, try to put each drive on a different cable - HDDs are easily the unit that puts the most constant stress on the IDE bus, so it is good to avoid having two units fight over the same bus line.
My two cents,
Pascal.
