I've been told repeatedly that you can get the Nikon software to work on anything up to Windows 7. I've never tried to jump through any of the hoops for doing that though as I already own a copy of Vuescan and it just works as is (Nikon LS-2000).
Your LS-20 is supported
http://www.hamrick.com/vuescan/nikon_ls_20.html and in fact the author of Vuescan recommends Nikon scanners if you ask what kind of scanner to buy. The only down side to the LS-20 is the lack of IR dust removal, which I find invaluable. IR dust removal is useless on a traditional black and white film though as there's too much silver present to let the IR pass through the film at all.
SCSI cards on the other hand can get tricky. Most will work with XP or Vista, but some don't have drivers available (buying used as it's inexpensive) and even fewer have drivers for a 64 bit OS. But I'd rather chance a PCI to SCSI card than try your luck with a USB to SCSI adapter.