Your power supply should be fine, I am running my gtx 460 on a corsair 400w. Overclocking a gpu does not lead to a large increase of power consumption unless you volt mod the gpu (which is not recommended, don't worry about it).  

You should be fine overclocking with the tools evga provides, Precision and OC scanner. Start out with precision, up the "shader clock" to 1400 for a start. This will bump the core clock up to 700 as the "core clock" is locked in a 1:2 ratio with the "shader clock". Run OC scanner's stress test for 5-10 minutes, and make sure there are no artifacts. Keep increasing the Shader clock by 50 and running the stress test until it artifacts. When it finally does, back the shader clock down to the last stable setting.   

You should get up to 1600mhz shader no problem, I myself have broken 1800mhz.

Your PSU is just scraping the minimum requirements for the 460 (recommended 450 watts). I've been using Corsair for awhile now, and they have always been rock solid.

Assuming you are in the US, $80 will get you a VX-550 which is more than enough for 1 card, leaving you with plenty of overclocking headroom. I just upgraded from one of these and was very happy with it. Sorry, you can't have it; it's going in my HTPC :P

http://www.newegg.com/Pro...17139004&Tpk=550vx

For only $10 more you can get a TX-650 which is a better unit, and which can handle 2 x 460 cards should you ever decide to upgrade to a pair of them and an SLI motherboard.

http://www.newegg.com/Pro..._-17-139-005-_-Product

Grats on your purchase :)