1. I need a detailed definition of "consumables" versus "durables" as
it pertains to understanding consumer spending. I need to understand
the rules for how something gets defined as a consumable versus
durable. For example, is a PC consumable or durable? How about a DVD
player? How about a DVD disc?
2. I need a detailed definition of "discretionary income" as it
relates to consumer spending. For example, what exactly is counted as
part of non-discretionary income? What if a consumer buys luxury
versions of those non-discretionary items? Do they become non-discretionary?

I think you have been on a short Economics course.

'Consumer Durables' is definitely the word for DVD players,
automobiles and refrigirators.

Beers, steaks and recreational drugs are normally 'consumables'

"discretionary income" is an odd concept
- you probably mean 'discretionary expenditure'
  - like what you can spend per month after paying things you have
little choice about after mortgage, city taxes, utilities, basic food
and alimony.

"discretionary income", if you really mean that, is really 'marginal
income', in other words money you could earn if you felt like it.

Somehow I do not think that you meant "discretionary income".

Discretionary expenditure is probably what you are after.
This opens the door for someone else to say whether 'investment' in
consumer durables is necessary or discretionary expenditure.
- and if so how discretionary is quality

I know an old guy who says 'I am too poor to buy cheap shoes'

According to the Bureau of Economic Analysis website, durable goods
are defined as "tangible products that can be stored or inventoried
and that have an average life of at least three years."

Three years is the standard time, and it also matched the Census
Department's definition used in calculating manufacturing inventories
and orders. The BEA looks at goods in the context of spending.

A 'consumable' (your term) is more precisely known as a nondurable
good, which is any tangible product with life of less than three
years.

As to who decides the lifespan of a product... I couldn't find that.
There has to be someone somewhere who makes the determination on how
new products are classified. For example, when the iPod came out,
someone had to decide if that was a durable (> 3 years) or nondurable
(< 3 years).

In your second question, I think you might be looking for the term
'Disposable Personal Income.' DPI is defined as all personal income,
less current taxes.
You can see a detailed breakdown of this here:
http://www.bea.gov/bea/dn/nipaweb/TableView.asp?SelectedTable=58&FirstYear=2002&LastYear=2004&Freq=Qtr

Disposable Personal Income (DPI) is either spent (Personal Outlays) or
saved (Personal Saving). In the US, almost all is spent, and the
lion's share of Personal Outlays goes into Personal Consumption
Expenditure (PCE).

PCE is broken down into durable, nondurable and service spending. Of
those three, durable is the smallest.

See more on PCE here:
http://www.bea.gov/bea/dn/nipaweb/TableView.asp?SelectedTable=81&FirstYear=2005&LastYear=2005&Freq=Month


Based on your question, I would guess that PCE amounts would likely
give you the data you need. PCE represents Personal Outlays after
interest payments (not really discretionary) are taken out. Of course,
you have to move higher up the chain to get to the save vs. spend
decision.

Luxury/Non-luxury doesn't really enter into the equation. You can
loosely say that luxury items are more likley to be durable (jewelery,
yachts, art), but expensive food and drink is still considered
nondurable.

Thanks for your comment - it helped me clarify my thinking. 

Discretionary expenditure is the concept I'm trying to get a better
understanding of.

Think of it as 'beer money'