When I said all GTX cards I may have been a bit too vague
The really old nvidia cards also featured GTX in the name, then nvidia went back and reused the naming with the new series of cards.
The card ranges you are looking for are as follows, roughly best first.
GeForce GTX TITAN X
GeForce GTX TITAN Z
GeForce GTX TITAN
GeForce GTX 900 series
GeForce GTX 800 series (this series is mobile only)
GeForce GTX 700 series
GeForce GTX 600 series
GeForce GTX 500 series (old now but ok as min upgrade)
GeForce GTX 400 series (old now but ok as min upgrade)
Each series normally goes up in increments of 10 so in the 700 series you have the following cards. 780, 770, 760, 750, 740, 730, 720, 710. The general rule of thumb is that the higher the number the better, until you get to the TITAN cards which are the best cards you can get and really are over kill for any current games.
Avoid cards with any other names as they are outdated.
After each of those series names you may see two things these are as follows...
"M" for example "GeForce GTX 780M", these cards are normally only used in laptops and are smaller more compact versions of the non "M" cards, I would never suggest getting one of these on desktop machines as they lose a lot of performance to fit into laptops.
"Ti" for example "GeForce GTX 660Ti", put short the Ti an updated version of that card which often consumes less power while providing better performance.
You then also have AMD cards which I could explain also if anyone really wants and AMD card but in my experience nvidia is better while AMD is the cheaper option.
You may also have Intel cards which are all integrated and are generally awful performance-wise, I could explain these as well but there are so many different versions and they all have the same name and perform better based on your CPU version, which makes it really hard for anyone to tell what GPU they really have.
I have a GeForce GTX 780 on my GameGuru machine which is now about a year old and runs every game I want it to at max at over 60FPS, it is the best upgrade to a GPU I have ever made and is also not that expensive now it isn't a brand new card. Anyone looking to upgrade would do well to get one, I had a GeForce GTX 660Ti and that was great but was outdated very quickly when the 700 series came out with much better performance.
For GameGuru anything from the 600 series or above will be smooth running. However please remember that GameGuru is very CPU heavy and as such a new GPU does not guarantee a fast gaming experience.
Hope that helps
Windows 7 Pro, Intel i7 3.8 GHz (Passmark: 9021), 16GB DDR3, NVIDIA GTX 780 4GB Superclocked (Passmark: 8056)