CASE: ROSEWILL CHALLENGER
Motherboard: ASUS M4A89GTD PRO/USB3
CPU: AMD PHENOM II X4 965 BLACK EDITION
GPU: HIS H545H1G Radeon HD 5450 (Cedar) 1GB
HHD: Western Digital Caviar Black WD6401AALS 640GB
MEMORY: 8GB PNY MD4096KD3=1600_X8 XLR8
OPTICAL DRIVE: LITE=ON IHAS424-98 LIGHTSCRIBE
OS: MICROSOFT WINDOWS 7 PROFESSIONAL 64 BIT
I have also read that the "ideal" is for the operating watts be 50% of the PSU. In other words...if your system operates at 300W the PSU should be a 600W. That said, do I NEED a 700W or is it better to just get a 450-500W?
Don't forget to include things like capacitor aging which should be low for a hi end PSU and higher for a low end.
While it is true that PSU's are the most efficient at 50% load, that does in no way mean that you "NEED" to operate at the BEP (best efficiency point). Your 650 watt PSU, may be say 90% efficient at 325 and 88% efficient at 500. That being said, given current price structures, I would never buy a PSU less than 650 watts.
I'd look for an Antec EA-650 / Antec TP-650 or Corsair TX 650 .... 500 watt PSU's of comparable quality are usually only around $10 cheaper.....most are not of comparable quality even if they are the same brand....for example, while Antec EA/TP/TPQ/SG/HCP/HCG are all good to very high quality, the Neo/Basiq lines are cheap OEM level units
A quality 450 to 500w will be more than enough, but...
What is the application of the computer? While a 5450 was a good choice for me (I needed 3 video outputs, and I don't play games) The 890 has pretty good IGP, since you only have a single monitor listed, you may be better off using your on board graphics or getting something more robust.
Running flat out, that PC likely won't pull more than 200W. Sitting mostly idle, 75W is more likely. With no plans for a more powerful video card, a 380W Antec Earthwatts would be my choice for this machine, and I'd expect it to run happily for years.