what games at what resoltuion at what quality level?
look at some gpu charts (hint: anandtech has some nice benchmarks) to see where your card rates. also see if your cpu fits the minimum for the games you wish to play.
i can tell you for a fact you should be using 2x4gb sticks not 1x8. you want dual channel (two sticks) not single channel (one stick)
personally i'd suggest upgrading your budget unless you're running either low graphic levels or old games.
running a 2 x 4gb ram kit would allow it to run in dual channel mode which is faster. apart from that their isn't much more you could get for $400. If possible it would be better to get an am3+ motherboard and cpu, as what you have cant be upgraded to much better than what you have. http://pcpartpicker.com/p/1Beb4
good build. if you had a few more $$ for a better cpu like an fx 4300 or 6300 that would be ideal, but with the build selected at least you have the option to upgrade your cpu when you have more money.
How about check some used components. 400$ is a low budget. Use some used components and mix some new ones. Thats the best option i can think with good specs.