The Minolta scanners seem to have a lot of problems.  The last version
of the 5400 was plagued with severe mechanical problems - a pity,
because it was otherwise an excellent scanner.  

The support from Sony is not said to be good, however I heard a couple
of ex-Konica Minolta technicians have set up a new service facility in
or near Milton Keynes.  Unfortunately I have no further details, but a
call to Konica Minolta in Milton Keynes might help - they still exist,
because they still sell photocopiers, and they might be able to put
you in touch with their ex-colleagues.

Your best option in new scanners is Nikon.  Canon scanners are no
longer made.  They were always the poor relation to Minolta and Nikon.

Three Nikon scanners are still listed, the Coolscan V for 35mm, with
4000 dpi and 14 bit processing, the Coolscan 5000 for 35mm, also with
4000 dpi but with 16 bit processing, and the Coolscan 9000, for 35mm
and medium format film, also with 4000 dpi and 16 bit processing.  The
Coolscan V is USB 2.0, the others need a FireWire 1394 card in your
PC.

They are not cheap, but they are superbly made and ultra-reliable.  If
you are happy with the Scan Dual IV, the Coolscan V will be a
substantial step up with its 4000 dpi resolution.  

The Coolscan 5000 is a lot more exoensive and the extra two bits (16
bit as against 14) are only needed if you want to extract the absolute
maximum of shadow detail out of your slides.  It is a very fast
scanner, with a slide taking less than 30 seconds.  

The 5000 will also accept a 50-slide capacity feeder which makes
scanning of large numbers of mounted slides much easier and quicker.
It will also accept an adaptor for roll film, meaning you can scan a
whole 35mm film in one operation.  However, these accessories are
expensive.

If you use medium format film, the Coolscan 9000 is a must.  It also
scans 35mm film, but is slower in use than the Coolscan V and 5000.

Nikon scanners are selling in only very small numbers now, and there
are rumours that production has ended.  So don't delay if you want to
buy a new one.  Shop around online for best prices, as there is a huge
variation.  Prices in New York are dramatically cheaper than here, so
if you plan a US visit in 2007, buy one while you are there.  Even
paying import duty (3.5%) and VAT (17.5%) you will save money

Good luck!