I was thinking about going TRI sli 480s, and was wondering if the kill-a-watt devices would actually tell me if I'm close to overloading my PSU. Is this true? Can they be used for this? It can't figure out amps on the 12v rails correct?
No, the device measures the total wattage that the PC is drawing. You could assume that all or most of the wattage is drawn by the 12v rails. Subtract a bit for psu inefficiency, and you could get some idea of the amps used.
I do not think tri GTX480 will be very useful, possibly so only if you are gaming with a 2560 x 1600 monitor.
If you are willing to drop that much cash on your graphics cards, a top psu will be a drop in the bucket more.