312 MB free Ram is plenty. When RAM gets below about 40 MB, Android starts shutting down apps that are running in the background which means they have to reload when you switch to them. Both shutting down and restarting apps takes time and uses CPU, which makes the phone feel slow.
I don't start to worry about RAM with any Android device until it gets well below 100 MB.
As a point of reference, my Motorola Photon, which has 1 GB of RAM, has about 460 MB free at startup and between 120 - 300 MB free after I've been using it a while and it runs fine.