First and foremost, what is the make and model of your PSU? That makes a big difference.
A 4460, 290X with 2 sticks of RAM, 1 HDD, 1 optical drive and 3 case fans will pull 485w under max load, which is slightly out of the ideal range for a 600w PSU but for a quality unit, it's just fine. Most of the time your system *will* be in the ideal range, even while gaming, as 485w is peak power.
I don't suggest adding any heavy power draw components, though (an SSD would be fine, they draw less that 3w).
Cheers!