Hmm. I'd undo the fans, and try to boot. I think youre at about the limit of the 2 psus. Both are about the same overall size.
There are 2 things to look for in power supplies. The Watts and Amps.
The 500W psu has 2 rails with 18Amps... so thats about 32Amps if you compare to a single rail one. You want as much ampsx as you can on a psu... Its like a water hose, you have psi and flow rate. If you have 10gal/min but 3psi youre not gonna get the water far... But 10gal/min anbd 100psi youre gonnza chop a person in half.
Heres a good example, this Antec 500W has 2 rails but 22Amps each.