Why do you want a modular power supply?
The main advantage to a modular supply is, you can reduce system clutter & improve air flow by running Just the wires you need. This is a big deal in a smaller, minimalist system. But, if your running a loaded up system with dual GPUs you'll end up using just about all the wires any way.
The second think I've seen is sometimes the modular supplies are bit quieter / more powerful. Here is one example I came across....
You get silence to 525watts with the 750HX
http://www.corsair.com/power-suppl [...] x750w.html
Here is a BIG difference the 750TX is Only silent to 375 watts!!!! (so, there is obviously more going on between these models than Just modular vs non-modular...)
http://www.corsair.com/power-suppl [...] x750w.html