Look at using HuffYUV lossless compression, which takes up about 30-40GB/hour of footage.
Granted, that still won't last long if all you have is 80GB total drive capacity. Not the mention writing to>from the same drive takes longer, as does using the same drive where Windows is installed (along with the temp/swap files).
Anyway...
- How many PCI or PCI Express slots are open and available in the computer?
- Any Firewire ports?
- What's your total budget?
- Do you live near a Microcenter or Fry's Electronics?
- Is there no IDE in this computer?
I don't think I've ever seen a SATA 80GB drive -- seems so tiny. Must be an OEM computer? (Dell, HP, etc?) Smallest SATA I've ever owned was a 400GB.