Is it possible to install Windows NT onto a second hard disk whilst leaving the first disk (in my case with win 98) untouched?... and boot to NT in a dual boot?

Yes - but you will want to use a boot manager to stop you from messing up your HD's.

Or you could install win2000 which has a better way of dealing with multiple OS. Install it second and it will recognize the first windows OS - then it will keep everything you install in win2000 on the proper partitions (e.g updates win2000's registry when installing software as opposed to the C.

Or you could use a Hard drive caddy to replace the HD's

or.....

You could install as many different Linux distros as Hard drive space you posses with no hassel.

I would remove the hard disk with win98 on it, put the empty one in alone as the primary, install NT. then put it back together and install a boot loader to the MBR.

You can, but like Cragstar said, you will need to get a boot manager because NT cannot read FAT32 file architecture and therfore its built-in boot manager is not a very good one.

You can use XOSL which is a really cool, free boot program. Easy to configure with many advanced features. you can find it at http:///www.xosl.org

Also, if you still Linux installed then you can use LILO.