It is always a good idea to use a solid PSU as the base for your system, however in this case the OP is replacing his current GPU with one that requires LESS power, so as long as nothing else is changed in the system there will be less strain on that poor little PSU and not more.... hence no kaboom
Yes you do make a valid point but personally (I am paranoid) I find that power supply like a ticking time bomb. You could use that power supply but I would not say it's the best, I hear that unit has poor efficiency and lacks certain protection features (I read another thread about it).
Anyways to the OP if you want to check your PCI Slot version we are going to need to know your motherboard. That being said PCI-E 3.0 cards work fine with PCI-E 2.0 slots.