HP z800 NEWB

hello all I have a Z800 with 2 IntelX5570 2.93.  Currently with 24gb.  I am getting the fatal DIMM error so am going to swap out the 24gb with 48.  Question is since im getting in there should I upgrade the processor?  Also I do some light gaming via Steam, can run the unreal games without issues but other games wont load.  Any advice on upgrades would be appreciated.  I have looked at the you tube video you posted just wondering since its been a few years if there is an ideal cost effective solution any one could point me to.

Thanks!