Its a 600 watt psu. I have extra power connectors for it also. Could you explain PCI-E power leads? Do you mean the standard connectors from the psu? What is this also, 24a on the 12v rails
Tht GTX460 needs more power than the motherboard pci-e x16 slot can deliver.
To get the extra power, you need two 6 pin connectors from the psu to the card. They can deliver 75w each.
If your 600w psu has at least two such connectors, you should be good.
Some older and cheaper psu's advertised high wattage for marketing purposes, but delivered the wattage to the 3.3v and 5v parts of the psu which is easy to do.
It is the +12v area that the cpu and graphics cards use, and that is where the power needs to be available.
Look at the data plate on your psu. It should list the amperage available on the +12v rail/s. There may be 2 or 3 such rails.
The total amps available is rarely the sum of the individual rails, and is usually less. Modern psu's will state also the combined +12v amps available. Sometimes that is stated in watts, so to convert to amps, divide by 12.