Going from 4 to 8 GB of RAM isn't going to help gaming performance, but if you run a lot of programs at once it could potentially help. The 7770 will significantly outperform the 5400k's built in graphics. You will not be able to use both at the same time, so you would want to disable the onboard graphics.
Thanks for the reply, The 7770 will work just fine right ? The purpose of this is for FSX and I know that it require a good CPU such as the i7 but will this upgrade help at all ?