I didn't get the numbering also till someone told me how it works. The first number is the series and after that it's the level of the card you could say. So the 750 is the new line of cards on the low end, and the 780 is at the high end. I don't know what the ti stands for, but they are the next one up for that card. So you could say the 750ti is a 755. For AMD/Ati it's the same from what I have been told. The HD7xxx is the older line that was replaced by the R7 and the R9 lines. The R7 seems to be the lower-medium end cards and the R9 is the higher end cards. The 290x would be the highest level card you can get from them.
I have read that for AMD/Ati some of the newer cards on the lower end are really just the older HD7xxx cards rebranded. Also after all the x and ti lines you have what the different manufacturers have made. With EVGA for the 780ti line they have the Superclocked, Classified and the Kingpin. All are basically a different fan and heat sink than the stock and factory overclocked. The Kingpin does have some different chips. I have read that the memory on it is from Samsung and not from the company that makes the memory for the other cards from EVGA. Some of the specialty cards don't come out when the rest of the cards do and not all lines will get them.
For the 750ti EVGA does have other lines like they do with the 780ti. I'm sure other manufacturers have done the same.
Seeing how your card runs games pretty well it sounds like you're in the same situation I'm in. You want a new card however it isn't needed and it seems like it's for the same reason. I ran the beta on a 5750 (2 generations older than your card if I'm right) and it worked fine. I think holding out is a good idea and read some reviews when you do get one. Compare the specks to what you have now as you may find that the really low end isn't much better than what you have. If you go with nVidia I suggest buying an EVGA branded card. All I have ever heard is good things about them and they have a step up program.
http://www.evga.com/support/stepup/ if you buy the card and in 90 days from when you bought it a new one comes out that is better and you decide you want it you can pay the difference and get it.
I hope this has been some help for you to understand what the numbers are for the cards.