Single rail or multi rail doesn't make a difference. That's Where the power comes From, not Where it goes To.
In a 2 rail psu, power is split into 2x 30Amp rails, then is rejoined at the main buss bar to equal 60Amp total. In a single rail psu, it uses a 60Amp circuit instead. Both psus have 60A total on that buss. (in general terms). Multi rails are cheaper per actual component since they don't require larger components of higher value, but overall can cost more because they use more components.
DC voltage is Direct Current. It's supposed to be a dead straight line. AC voltage is Alternating Current, the line goes up/down like a wave. A bad quality psu will not convert AC voltage into DC voltage very well at all, so the DC line is still very erratic, tons of small ups-downs constantly, which makes stability nearly impossible, burns up gpus and motherboards and ram quicker etc. A good quality psu has far better voltage regulation, the DC looks more like the straight line it's supposed to be, makes stability easier and better, helps components live a longer life.
Bad quality psus have little to no protections. If power load is too great, the circuits over heat, too much amperage, you get a fire in your pc. Bad quality psus lie to you to make themselves sound better. A 6pin pcie is rated for 75w at 12v. If you see a 750w psu with 1x 6pin, where is all that power? Where is the remaining 675w going? It's not going to the pcie/vga guaranteed.
Any halfway decent 500w-550w will have at least a 6pin + 6+2pin, total of minimum 225w just for the gpu. Doesn't really matter if that's 1 cable or 2 cables unless it's a really cheap junk psu that doesn't use heavy guage wire.
Bronze, Gold etc is an Efficiency rating only. If your psu is Bronze, it means 80% or more of the power pulled from the wall is converted to DC. Meaning if your pc has a total power use of 240w, with a Bronze psu you pull 300w from the wall. Only 60w is lost as heat inside the psu. With a White or N (no color) rated psu, that efficiency can be as low as 50%, if your pc used 240w, you could pull 480w from the wall, 240w worth of heat lost inside the psu. That = good chance to start fires because components get far too hot when generating that much heat. The psu looks more like a toaster on full burn.