I think it it important to define "extreme" in cooling. In most
environments where computers are used you can ususally
go 5-7 degrees C below ambient before condensation can
happen. With a properly temp controlled water chiller this
is pretty easy. Far easier, in fact, than all the issues of
placing a Peltier in your PC.
You can actually cool quite a bit below ambient if you have
low humidity. Take a look at
this chart.
Given your conditions, if this is "extreme" enough for you then
a water chiller is a fine solution.
Note: many people have tried to build a chiller
with Peltiers. Most failed.
If you want to go in this direction copy the design
of commercial units.