To close this thread off and 'back up' IO's post - I finally plucked up the courage and bought a new SATA 160GB. I am now fully 'cloned' and booting off that drive.
Since there appears to be a bit of 'confusion' around in Google I thought I would document the process in case anyone else is unsure.
1) New SATA added to existing multi-boot mobo which has 1 SATA already (SATA drivers had previously been installed from mobo CD)
2) OS Disk cloned to new SATA
3) Now the fun starts - of course it will not boot. At this point, out came the very best bit of systems/diagnostic software I have ever had was used - Hiren's Boot CD, transferred to a bootable USB stick. The beauty of this is that as one who is known to be deficient in command line programming, I was able to use its inbuilt 'mini-XP' to open my machine in an XP GUI. Thus I was able to use Disk Manager/Partition Magic etc on my existing drives and see exactly what needed to be changed in boot.ini.
4) Change done, and it re-boots normally and re-aligns the drive letters.
5) Tentative, nervous 'disable' old system drive
6) Here I am.
As far as I can see all is functioning.
Now I have a spare 80GB IDE - time for W7.................?