You'll know if your motherboard is incompatible with your gpu if you try to plug in the gpu and the hole is completely the wrong shape. You're upgrading from a gtx460 so you probably have a pci express port which is basically the standard and has been since, like, 2006? The bigger problem might be that the card doesn't fit in your case, but 460s are already pretty huge so you might not have to worry.
If your power supply doesn't have enough wattage your computer will still work, it'll just be really choppy and a little thing will pop up that says "Hey you don't have enough power!" Once I installed a gpu and completely forgot to plug the fucking thing in entirely and this was pretty much the result. If this happens I guess reinstall your 460 to hold you over until you can go buy a new power supply.
You don't have to worry about hurting yourself or your computer during gpu installation. Just don't rub your feet on the carpet and then zap the motherboard with static electricity like a jerk and you'll be fine. It's insert-tab-A-into-slot-B.