So, you've got a WD6400AAKS, E7200, HD 4850, Antec 900, 22" monitor, and you're thinking of a 500W PSU. That's powerful enough (minimum recommendations 450W.)
If your MB supports Crossfire, you should get something 550W or larger.
Can you spend $20 more and get the PC Power & Cooling 610W? That's a tier 1 PSU (the smaller PC P&C's are tier 2), supports Crossfire, has a better chance to be reusable with your next PC or video card. At $60 with free shipping it's an incredible price.http://www.newegg.com/Product/Product.aspx?Item=N82E16817703005