nvidia recommends 500w. i think those recommendation are based on ordinary PC which means no OC for cpu and gpu and no SLI. if you put OC into consideration maybe you want to up the the psu requirement a bit like using 600w psu. if you want to use SLI in the future then much bigger PSU is definitely a must
I wouldn't endorse running it on a 450w psu, even if it's a very well designed one. What brand is the PSU (if it's not a top quality brand, you should definately not use it, it could damage the PC, if it's good quality and has good protections it maybe safe to try but I would expect it to be unstable).