Need a 750w power supply after upgrade?

Going to upgrade my system to Pentium® Dual-Core Processor E5200 with Asrock 4CoreDual-SATA2 R2.0, 2 gb ddr2-667, ati radeon x1650 pro 512 mb, 300 g hd. I currently have a 450w power supply.

Do I need to buy upgrade my power supply to 750w?

Even a poor quality 450W will work.

A good 400 or 450 would work just fine. As for the comment above, a poor qualilty PSU is never a good choice, since even loaded well below spec, they can have out of spec voltage or ripple and damage other components.

what is the PSU....

100W is a bit of a low estimate (though I agree a good 300 would be fine too). A friend has an E6600 and an HD3870 and 3 hard drives on a good 375 and it's been flawless for several years now. The problem with poor quality PSUs isn't just overloading though. Here's a poor quality PSU's (specifically, a 350W unit) 12V ripple at a load of just 50W:
http://www.jonnyguru.com/modules/N [...] 1-12V1.jpg

That's already mediocre regulation (though within spec). The same unit reached 3V of ripple (out of spec by a factor of 25) at a 235W load. You want a good PSU, even if running well under spec. It's true that the PSU reviewed in that test probably would handle the load and not blow up given the OP's system, it would do so with such spectacular failure at regulation that the rest of the system could die anyways.

(for more details on that spectacular failure of a PSU, look at the jonnyguru review "Death of a Gutless Wonder here)

E5200 ~30W
X1650 Pro ~44W
Mobo, HDD, RAM ~40W