your psu will be fine . The HX 620 has been tested to 700 + watts and remained perfectly stable
I dont know how demanding the new game will be but , I think you will need to update the processor as well as gfx card .
Either a 9550 quad or e8400
Also check out the GTX 275 . Its a lot cheaper than the 285 and performance isnt that much different
You may also get better performance at a better cost by switching to a pair of ATI 4850's in crossfire which perform about the same as a GTX 285 . A pair of 4850's should cost about $180 for bothhttp://www.xbitlabs.com/articles/video/display/sapphire-radeon-hd4850-cf_7.html#sect1
Or 4870's if you have a big enough budget .........