or AMD build :) You could get a 720 tri core with a chance of un-lockable 4th core for the same price.
the 550 watts cut it, but there is not much head room for upgrade. Since you did not upgrade from a P4 for all this time, one might think you dont plan on upgrading soon. So PSU headroom has no real value for you. This might help you, it is sthe official recommended PSU wattage calculator from corsair :
Welcome to Corsair :: Power Supplies
corsair even say that their 450 W is good enough for 1 hard drive, 1 Gtx260 and one dual core. Same if you go AMD tri core way.