I've got about 60A of 120v equipment to run in one room (at times, all
of it simultaneously). Currently there's only one 20A circuit feeding
the entire floor. So I was thinking of installing two 30A circuits
and one 20A.
this is for my art studio. here's a partial list:
electric griddle 12A
electric griddle 12A
heat stripping gun 12A
1 or 2 space heaters 12A each
A/C in summer 8A
computers 4A or more
tools 15A

It sounds like you have a lot of high current appliances. I personally
would run a separate circuit for each of those.

Based on your list, I would install eight 20A circuits, using 12/2 cable,
and regular 20A recepticles. Your computers could share the lighting
circuit if you wish.

Depending on your usage patterns, you may be able to combine the stripping
gun and "tools" on a single circuit too. For example, you probably wouldn't
be using the stripping gun, a drill, and a dremel tool all at the same
time. Those could share a circuit if you wish.

Basically, if you use an appliance that draws 10A or more and is on
continously (i.e. the griddles), you should have a separate circuit for it.
You could probably get by with 15A circuits for most of your appliances,
but it wouldn't cost much more to run 20A and would give you greater
capacity.

If you have balanced loads (i.e. two matching griddles), you could combine
a couple of circuits with a single 12/3 cable, sharing the neutral line.
Just make sure the two hot wires are on opposite legs of the incoming 240V
supply. This would reduce your cable runs somewhat, and is frequently done
for similar situations like kitchen wiring.

If your breaker panel is a long distance from your studio, it may make more
sense to install a subpanel in the studio, then run each circuit from the
subpanel. This would ease later additions too.

Anthony