Where is the significant point of diminishing returns on hi-end turntable?

I think the point of diminishing returns moves along a spectrum, depending on the rest of your system, how "dialed in" it is (which is something that takes time after living with various components and getting the most out of them) and the nature of your record collection.
In the '80s, I had a Well-Tempered Turntable which, at the time, gave a huge amount of performance for the money and it was fine with a first class cartridge over a system with limited bandwidth and dynamics (Quad electrostats). When I moved to a bigger, wider bandwidth system, I changed to a more conventional, substantial table- a Kuzma Reference with Triplanar arm and a selection of good cartridges. But the revelation, for me, was upgrading that Kuzma for the XL (a high mass table) and the same manufacturer's air bearing arm (using the same cartridges), which made the following differences:
less sense of a turntable playing- no 'halo' around the sound or other artifacts that tell you a reproducing mechanism is in play;
deeper, more natural bass- sounded more like instruments than just low frequencies.
This is most obvious on good sounding records- not the "audio porn" which can sound spectacular for "demo" purposes. Once you start playing music you like rather than stuff that shows off your system, you'll hear a big difference between different records and different pressings of the same record- some will sound far flatter and lifeless in presentation than others.  When you listen to a wide variety of different records, you get a different baseline than just "demo" testing gear on a few impressive sounding records. I think this is where the differences in front end start to be revealed: not differences in overt colorations, but the ability to extract more information from the record without having an analytical sound (the reason, I think, why most vinyl lovers cling to the medium). But, the big variable, all other things being equal (which they never are)* is often the records at that point. 

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*The turntable/arm/cartridge/phono stage combination is probably the hardest to audition in the process of buying, since there are so many variables and so few opportunities to make direct comparisons. 

You'll never be satisfied.  The guy in the Memorex ad is an illusion.  There are better turntables, cartridges and phono stages then what is being recommended for you by the dealer.  If you know they exist, then you'll want to experience them.  That's the real game.

At a practical level you'll also need a very good equipment rack.  Then there's RCMs and after that the endless chase for the best sounding pressings.  It's my observation from reading the forum's that the difference between experienced audiophiles and neophytes is that the old hands know when enough is enough.  You'll probably have to learn that lesson for yourself.

whart, excellent discussion of the journey! What you describe very closely matches my experience, also.

mmporsche--That is a great value that Rushton mentioned for the Walker table. IMO, not sure there would be a better value out there for that type of price, but that is just another opinion. I do also agree with many others who commented on the weak link approach to your system. I have found many vendors over the years who let you "use" equipment  to test or try in your own system. Nothing like doing and A vs B in your own setup. I wont mention names as some people on the site dont like others getting free publicity, but i really enjoyed comparing in my own setup and room. Also, as Rushton highlighted, the selection, care and cleaning of your vinyl becomes a necessity once you get to the reference level. Nothing like losing the benefits because your records are dirty, worn, etc. 
The main advice is enjoy the ride cause the trip never ends!

I hope you are relatively young, and I hope you have the wisdom to pretty much ignore all this advice.  Except mine, of course.  Start low and slow and listen carefully at each step in your system development.  Add and then take away in order to get a bead on what does what.  It takes several years to gain a command of the hobby and of how to get where you want to go, and only you can do the work. It doesn't hurt to learn something about electronics and acoustics along the way.  Read books.

Basic questions: Do you think you prefer tube devices or solid state? Turntables typically employ one of 3 types of drive systems: Belt, Idler, and Direct.  Each of these types of drive system imparts a certain characteristic sound; decide which you prefer.  Etc.  I would definitely NOT start out by investing in expensive power cords, as you mentioned above.  Just make sure your power cords are better than lamp cord and are of relatively thick gauge (For example, Loew's sells 12-gauge power cord that many say sounds great for very little money), and then worry about the rest.