The 710 is Fermi architecture, previously known as a GT620. The 730 is a Kepler, previously known as the GT640. I don't know which one exactly you mean but as far as I'm aware, all the new low end 700-series are rebranded 600-series. Note that the GT 430 was also a Fermi based architecture. So basically, Fermi is 4 generations old with the 710. They just keep rebranding it with minor updates at best.
Typically, the old midrange is cheaper than the new low end. While its true they can be slightly faster, with faster or more memory, wider memory bus and higher clock speeds and die shrinks, its generally not enough to be able to turn Madvr up a notch. IMO there isn't anything in the new low ends that would make it worthwhile over the older midrange. Maybe if they would come with half the power consumption but I haven't really looked into it.
I'm still at a GT430 and it runs Madvr with medium to high settings. No low to midrange 600-series or low end 700-series will do any better. Maybe a notch on one particular setting but nothing major and certainly not enough to make a visual impact.
If you're looking at building a new system, by all means pick the latest greatest but if you're looking at an upgrading a 430 or HD4000, I wouldn't bother and stick with what you have.