Actually...

You could connect one CD drive and the old HDD, and then once you've copied across the contents of the old HDD (i.e. your documents & music) onto the new HDD, then remove the old HDD and plug in the second HDD :)

So that would be:
1x new Motherboard
1x new CPU
1x new memory
1x new HDD

Remember, the advantage with getting a brand new HDD rather than rely on your old one will be that after 5 years, the failure rate of HDDs increases exponentially! Of course, new drives fail too, so you could also perhaps grab a $40 USB 'external IDE enclosure' and use the old drive to back up your new drive.