I know, casual gaming you say, but if you OC the G3258 with this setup to about 4.2-4.4 Ghz you'll be getting really good performance for 400 euros. It's what I would get no matter the type of gaming.
Good build by rcf I would agree the g 3258 is a awesome little dual core I have mine oc to 4.4 all day long but don't expect it to game on some games that require 4 cores.... But since your only casual gaming your good to go.