Rant: "The Length of a Novel", or "H.G. Wells' Swelling Word Counts"

I'm currently attempting to write some fiction.  For pragmatic reasons, I'm somewhat concerned about the length of my story in terms of saleability.  It seems that, for the most part, there are only markets today for long novels (published initially in book form) and short stories (published initially in periodicals).  I'm not sure where one publishes novella-length works (I noticed that Gollancz published its first-ever first-run novella recently).  There doesn't seem to be a set definition of what exactly makes a novel, which I find interesting, given that the RIAA and AFI have guidelines for full-length album and feature-length film, respectively .  The sentiment I see online is that people expect 80,000 words from a novel, at least from a new writer.  Established writers seem to get away with shorter novels when they want to.

The Wikipedia article on "word count" doesn't add a whole lot, although it says the Nebula science fiction awards only require 40,000 words.  Meanwhile, according to the same Wikipedia article, Jane Smiley says novels are usually 100,000-175,000 words.  I think she must primarily read extremely undisciplined works to come up with that figure.

H.G. Wells is one of my favorite writers of prose fiction.  I looked up the word counts of his first eight science fiction "novels".  Here they are, in chronological order, with thousands of words to the right:

1895 Time Machine (33)
1896 Island of Dr. Moreau (43)
1897 Invisible Man (48)
1898 War of the Worlds (60)
1899 When the Sleeper Wakes (68)
1901 First Men in the Moon (69)
1904 Food of the Goods (74)
1906 In the Days of the Comet (81)

The Time Machine would not appear to qualify as a novel by anyone's reckoning, yet it is frequently referred to as one. Also, isn't it amazing how uniformly the word count increases? Slightly less uniform are the diminishing returns, measured in legacy and impact, across the progression. By this I mean that most people have heard of the first four entries on this list, but not so much the last four. As his career progressed, Wells seems to have lost touch with his focus or discipline. I really enjoyed Food of the Gods but First Men on the Moon could shave most of the first quarter and In the Days of the Comet was mostly aimless nonsense.

So how many words does a novel need to have?