How much free disk space do I need to install Vista over XP Pro SP3?

I’m still struggling with migration from XP Pro to Windows 7. The problem is outlined here. Windows XP is running on an old 80 gig drive, and it’s becoming increasingly unstable. I ran chkdsk d: /r on it, but it’s still flaky. When I booted into that drive this morning, it worked fine for about a half hour then froze so solid it wouldn’t obey the three-fingered salute. I had to use a bus interrupt to kill it.

The 80 gig drive is about 94% full. If I can run chkdsk on it again and get it back to bootable condition, I can clean it up from a separate 1 terabyte SATA I have Windows 7 Ultimate installed on in the same machine. How much space do I need to first install Vista on the 80 gig drive, which can install over XP and pick up XP’s settings, then put Windows 7 over Vista, which will thereby transfer my critical email boxes into Windows 7?