If you play BF3 at 1680x1050 or less (and have a fast internet connection), you should be able to get good frame rates with mid/high settings. The Phenom II X4 850 is an older CPU without a level 3 cache, and you don't mention which motherboard it comes with. If it's a used system in good condition, maybe $400 max. Assuming it doesn't come with an O/S.
Thank you, i'm not sure what motherboard it has since its on ebay and thats all the listing says I will have to ask the seller thanks for your help