The new GeForce GTX 460 will draw a maximum of 13.3 Amps from the +12 Volt rail.
Your Antec NeoPower 550 power supply unit is able to provide 18 Amps from each of the three +12 Volt rails or 54 Amps combined. That will leave you 40 Amps on the +12 Volt rail(s) to power the remainder of your system.
Your current PSU has more than sufficient capacity for the new graphics card.
GTX460-1gb is a nice jump. It requires a 450w psu with 24a on the 12v rails. Your Antec has plenty to spare.
As a side note, the maximum output on a psu is not the sum of the 3 18a rails(52a). It is normally somewhat lower, in this case, it is 42a, which is still plenty.